Marian Ivan
email: marian.ivan@g.unibuc.ro

Crustal thickness in Vrancea area-Romania from S to P converted waves

Crustal thickness (CT) in Vrancea region (Romania) and adjacent area is investigated using 1294 S to P converted waves from the Moho discontinuity. A total of 269 local earthquakes in the depth range 99.8 to 171.1 km and recorded by 76 permanent and 46 temporary stations of the Romanian Seismological Network are used. Time difference between the converted wave and the direct P phase is corrected to a first order for epicentral distance and for the errors in focal depth, being finally inverted to CT. Greatest values for the Moho depth are observed for stations located in the Carpathians molasse foredeep and smaller values are observed in the Southern part of the Moesian Platform, for stations in the eastern part of Moldavian (East-European) Platform and in Dobrogea area, close to the Black Sea shoreline. In Vrancea epicentral area, an important CT variation is observed, from 42 km at MLR and 41.8 km at SIR, stations placed in the south-western part of the epicentral area, to 30.9 km at VRI, located above north-eastern part of the seismogenic volume. Stations CVO and OZU, placed in Transylvanian Basin in the proximity of the epicentral area, have CT values of 32.1 and 24.1 km, respectively. The results seem to support that a mantle delamination process is responsible for Vrancea intermediate depth seismicity.

Introduction

Vrancea zone is one of the most active intra-continental seismic areas in Europe (Fig. 1), with potential seismic hazard associated to a few intermediate depth earthquakes (1940, November, 10th, Mw=7.7, H=150 km;1977, March, 3rd, Mw=7.4, H=94 km;1986, August, 30th, Mw=7.1, H=131 km;1990, May, 30th, Mw=6.9, H= 91 km;1990, May, 31st, Mw=6.4, H=87 km). Surrounding Vrancea, the stations of the Romanian Seismic Network (Fig. 2) are placed in different tectonic units, i.e. Moldavian (East-European) Platform, Carpathian Arc and Moesian Platform. Attempts to explain the Vrancea seismicity cover almost the whole range of the actual existing geodynamic scenarios (see a synopsis in [START_REF] Mucuta | Constraints from Moho geometry and crustal thickness on the geodynamic origin of the Vrancea Seismogenic Zone (Romania)[END_REF]). However, both the nature of the sub-crustal seismic activity, which is confined to a near vertical volume, and the fine details tectonic structure in Vrancea and adjacent area are still open to various scientific debates [START_REF] Milsom | The Vrancea seismic zone and its analogue in the Banda Arc, eastern Indonesia[END_REF]. Frequently, a conspicuous phase located between P and S (Fig. 3) can be observed for events exceeding 100 km depth and recorded at epicentral distances less than 2°. The arrival time, particle motion and the frequency content (which is, at least in several cases, also intermediate between P and S) suggest that phase to be a S wave converted and transmitted as P on the Moho discontinuity (e.g. [START_REF] Båth | S-P conversion at the base of the crust[END_REF][START_REF] Smith | S to P Conversions as an aid to crustal studies[END_REF]. It is best observed at stations projected in the proximity of a nodal plane on the focal hemisphere, as it is also suggested by the relative amplitude ratios on synthetic seismograms obtained by the reflectivity method [START_REF] Wang | A simple orthonormalization method for the stable and efficient computation of Green's functions[END_REF]. Such converted phases have been extensively observed in various regions of the world and used to map the lithosphere-astenosphere boundary [START_REF] Sacks | The use of converted phases to infer the depth of the lithosphereastenosphere boundary beneath South America[END_REF], the crustal thickness [START_REF] Regnier | Crustal Thickness Variation in the Andean Foreland, Argentina, from Converted Waves[END_REF][START_REF] Narcía-López | Determination of crustal thickness benetah Chiapas, Mexico using S and Sp waves[END_REF]), the location of the upper slab-astenosphere interface [START_REF] Nakamura | Fine structure of deep Wadati-Benioff zone in the Izu-Bonin region estimated from S-to-P converted phase[END_REF], or the geometry of the upper boundary of the plates [START_REF] Ohmi | Seismic wave conversion near the upper boundary of the Pacific plate beneath the Kanto district, Japan[END_REF]. In this paper, the time difference between the converted phase (sp) and the direct p wave is corrected to a first order for epicentral distance and for the event depth by using the local velocity model (LVM) routinely used in earthquake location by the Romanian National Institute for Earth Physics (NIEP) [START_REF] Oncescu | Deep structure of Vrancea region, Romania, inferred from simultaneous inversion for hypocenters and 3-D velocity structure[END_REF]. By using a method to minimize the errors in focal depth, the corrected time difference values (sp -p) are inverted to evaluate the crustal thickness in (proximity of) Vrancea. The CT values are finally compared to the previous results provided by active seismic experiments in the area, by the receiver function analysis, by the inversion of the surface waves and to the values suggested by the astronomic quasi-geoid heights map.

Method

Let ij t be the observed minus computed (O-C) difference between the arrival of the converted sp phase and the direct P wave, manually picked at the j-th station for the i-th earthquake

  C p C sp O p O sp ij t t t t t     (1)
Especially for small epicentral distances, the errors in focal depth (routinely around 10-15 km) are shifting the computed sp-P time difference by a near constant value i C , which is approximately the same for all the recording stations (Fig. 4). Hence, to a first order, the difference

i ij j C t t  
(2) will be a characteristic of each j-th station, depending mainly on the Moho depth beneath the recording station. It can be evaluated from a weighted least-squared minimum

        NE i NS j i j ij ij C t t w 1 1 2 min , (3)
where ij w are the values of the weights, NE is the total number of earthquakes and NS is the total number of stations which recorded a certain i -th event. Vanishing the first-order partial derivatives of (3) with respect to i C and j t , it follows the unknown values of j t are obtained by solving the linear system

   NS k j k jk b t a 1 , k,j=1,…,NS , (4) with             NE i NS l il ik kj ik jk w w w a 1 1 /  (5)
and

              NE i NS l il NS l il il ij ij j w t w t w b 1 1 1 / , (6)
where kj  is Kronecker's symbol.

The value of j t has been assumed to be the same for stations with close location and/or in very similar tectonic settings. In such case, a representative station located in the proximity of the centroid of the whole group has been selected. For each (group of) station(s), an average earthquake location was evaluated. Its coordinates is the arithmetic mean of latitudes, longitudes and depth values of all the events recorded by that (group) of station(s) (Table 1). Piercing points to Moho of p and sp waves were obtained for every (representative) station and for the corresponding average earthquake using the LVM with the Moho depth at 40

km. The values of j t were interpolated with the minimum curvature method in a grid of 81 (latitude) by 100 (longitude) cells, being assigned to the mid-point between p and sp piercing points. For every (representative) station and corresponding average event, the value of j t was converted to Moho depth (M_d) by a trial and error method. The value of 40 kms assigned to M_d into the LVM and used in (1) was slightly modified until j t value was reached. All the other parameters of the LVM remain unchanged. For the 52 (representative) stations (Table1), there is a linear correlation between the evaluated Moho depth and j t values (Fig. 5) which was used to obtain M_d for the rest of the grid points.

Data collection

Three sources of digital waveforms available at NIEP Data Center have been used

in this study. The first one is represented by the recordings of the Geotech S-13 network (white triangles in Figure 2) during 1982-1997, which produced 61 earthquakes for the analysis. That network had a 50 Hz sampling and a common time base for all the stations (most of them being equipped with vertical sensors only). A second source, which provided 186 events for the period 1997-2007, June 30th, is the K2 network [START_REF] Bonjer | Source-and site-parameters of the April 28, 1999 intermediate depth Vrancea earthquake: First results from the new K2 network in Romania[END_REF] (black triangles in Figure 2), with a variety 3-component instruments at 200 Hz sampling and individual GPS timing. The third source was Calixto 99 experiment (Figure 6) which provided another 22 events. Several waveforms have been obtained also from GFZ Data

Center, for the Romanian broad-band stations. All the existing recordings have been checked for the presence of conspicuous P and sp converted phases. In each case, a weight (good, fair and poor) has been ascribed to each sp / p reading, depending on the signal to noise ratio and to the presence of signal on both vertical and radial channels (where available). Only earthquakes showing sp phases at two or more stations (at least one of them with fair or good weight) have been selected and processed according to eq.( 1)-( 6). The computed values have been evaluated using TauP Toolkit [START_REF] Crotwell | The TauP Toolkit: Flexible seismic travel-time and raypath utilities[END_REF] for the LVM having the Moho depth at 40 km [START_REF] Oncescu | Deep structure of Vrancea region, Romania, inferred from simultaneous inversion for hypocenters and 3-D velocity structure[END_REF]. Event locations are provided by the updated Romplus catalogue [START_REF] Oncescu | The Romanian Earthquake Catalogue between 984-1997[END_REF].

Results and discussion

The above estimations of Moho depth have been compared to the results obtained in the area from receiver function analysis [START_REF] Diehl | The crustal structure beneath SE Romania from teleseismic receiver functions[END_REF][START_REF] Geissler | Upper mantle and lithospheric heterogeneities in central and eastern Europe as observed by teleseismic receiver functions[END_REF][START_REF] Tãtaru | Research on lithosphere structure in Romania by using receiver functions method[END_REF]. The values in Table 2 suggest that the errors in the Moho depth estimations in this study are around ±3 kms, with a noticeable difference at station E25 where the difference is around 10 km. There is a good agreement too in respect to the Moho depth at shot points G, H, K, L and M of the controlled seismic experiment VRANCEA99 (Hauser et al. 2001). For the shot points D, E, F, R, S, T (VRANCEA99, VRANCEA2001), the Moho depth estimated from sp conversions seems to be underestimated by around 7-8 km. The discrepancies could be due to lateral variations of P wave velocity, especially in the crust, or a failure of the interpolation technique. The minimum curvature method is assuming a continuous, smooth geometry of a thin elastic plate (e.g. [START_REF] Briggs | Machine contouring using minimum curvature[END_REF], which could not be valid in the presence of a few crustal faults (see Fig. 1).

Furthermore, most observations at CFR station (which provided the Moho depth in the area of shot points R and S) are of low quality, especially because the arrival of P wave at that station is routinely difficult to be identified accurately.

Greatest negative values of j t are obtained in Carpathians mollasse for-deep and Focsani Basin, similar to the features suggested by the quasi-geoid heights (Fig. 2). The geodetic values are related to the variations of the Newtonian potential on the Earth surface, mostly as a result of both density contrast between the crust / upper mantle, and to the Moho topography. In the southern part of the investigated area (proximity of Bucharest city), a clear increase of the j t values is observed from North to South, suggesting a decrease of crustal thickness toward Danube. This feature is in agreement to the results provided by surface wave dispersion [START_REF] Raykova | Surface waves tomography and non-linear inversion in the the southeast Carpathians[END_REF]. In the Eastern most part of the Figure 7, An important variation of attenuation have been reported here by [START_REF] Ivan | Attenuation of P and pP waves in Vrancea area -Romania[END_REF].

Stations CVO and OZU, located in Transylvanian Basin in the proximity of the epicentral area, have CT values of 32.1 and 24 km, respectively, in agreement to the value around 27.5 at the temporary station S07 of CALIXTO99 experiment [START_REF] Diehl | The crustal structure beneath SE Romania from teleseismic receiver functions[END_REF] and to [START_REF] Raykova | Surface waves tomography and non-linear inversion in the the southeast Carpathians[END_REF] observations in cell 15 f (28.5-33.5 km). However, shot points U and W (VRANCEA2001 experiment) provided here a Moho depth around 34 km and a strong reflector around 27 km depth has been assumed to represent a limit separating the middle / lower crust by [START_REF] Hauser | Seismic crustal structure between the Transylvanian Basin and the Black Sea, Romania[END_REF].

Converted phases can be also used to provide an estimation of focal depth. For example, the value of i C in eq.( 2) for the 2005/04/04 event (Figure 3) is -2.07 seconds. It suggests that the assumed depth of 141 km (Romplus catalogue) is overestimated by around 20 km (see Figure 4), in agreement with the depth provided by other agencies (i.e. 120 km / SOF /, 121.1 km / CSEM / and 115.5f / NEIC /)(see details in ISC Catalogue). Considering the observations in Figure 8 and the LVM with the Moho depth at 40 km, the rms values for (O-C) sp-p time differences are 2.45 s (for the focal depth at 141 km) and 0.77 s (for h=121 km).

The rms value is further decreased to 0.53 s for the LVM with variable Moho depth and h=121 km.

Conclusions

Converted waves appear to be an useful tool for investigating crustal thickness in Vrancea and adjacent areas. Most results are in good agreement to the values derived by various techniques (receiver function, active seismic experiments, surface wave dispersion). Proper identification of the converted sp phase can improve the accuracy of intermediate depth event location (especially the focal depth), better constraining the focal mechanism too. Including such waves into local tomographic investigations can substantially improve the resolution of the derived models. The thin crust obtained from converted phases in the epicentral area might support the mantle delamination as a possible geodynamic mechanism for Vrancea seismicity (e.g. [START_REF] Gîrbacea | Slab in the wrong place: lower lithospheric mantle delamination in the last stage of the Eastern Carpathian subduction retreat[END_REF][START_REF] Sperner | Slab break-offabrupt cut or gradual detachment ? New insights from Vrancea region (SE-Carpathians, Romania)[END_REF][START_REF] Gvirtzman | Partial detachment of a lithospheric root under the southeast Carpathians: toward a better definition of the detachment concept[END_REF]Knap et al. 2005;[START_REF] Fillerup | Mantle earthquakes in the absence of subduction? Continental delamination in the Romanian Carpathians[END_REF].

geoid data. GFZ Potsdam is acknowledged for maintaining the Data Archive. GMT files [START_REF] Wessel | A global, self-consistent, hierarchical, high-resolution shoreline database[END_REF] have been used to prepare some of the diagrams. The author is grateful to Dr.

Rongjiang Wang (GFZ Potsdam) for making the reflectivity method code available. Comments and suggestions of Dr. Jiri Zahradnik and two anonymous referees definitely improved the manuscript.

jt

  values are decreasing from East to West, possibly because of the presence of a crust fault (the Danube fault), approximately located along the longitude of 27.5°-28°E, between latitude 44 and 45° N (see also the tomographic results of Fan et al. (1998)). The j t values in Vrancea epicentral area indicate a strong lateral variation of crustal thickness. At MLR and SIR, located in the south-western part of the epicentral area, CT values are around 42 kms. At VRI station, located 6 above north-eastern part of the seismogenic volume, CT value is around 31 km.

Fig. 1

 1 Fig. 1 Main tectonic settings of Vrancea and adjacent area (simplified after Polonic (1996) and Oczlon (2006). Crosses indicate the epicenters of the intermediate depth events used in this study. A histogram of the focal depths is also presented. North, Central and South Dobrogea regions are indicated by ND, CD and SD, respectively. Shot points of previous seismic refraction lines are indicated by circles (VRANCEA99) and squares (VRANCEA2001) experiments. Diamonds show the location of some CALIXTO99 temporary stations. C-O Fault and P-C Fault are Peceneaga-Camena and Capidava-Ovidiu crustal faults, respectively.

Fig. 7 .

 7 Fig. 7. Moho depth estimations in Vrancea and adjacent area. Piercing points to Moho of sp and p waves are indicated by circles and squares, respectively. Figures indicate the average values of Moho depth obtained by Raykova and Panza (2006) in a cell grid of 1° x 1°.

Fig. 8 .

 8 Fig.8. Vertical component recordings of 2005/04/04 event (see Fig.3), aligned to the direct P arrivals. Converted sp phases (manually picked) are indicated by vertical bars. Computed sp arrivals for the LVM with the Moho depth at 40 km are indicated by arrows (for focal depth h=141 km) and by diamonds (for h=121 km). Triangles shown the sp arrivals for the variable Moho depth model and h=121 km.

Table 1

 1 Coordinates of the used stations, the number of sp/p phases observed at each

	station, average earthquakes and j t values (with standard errors). Representative stations
	for a certain group are bolded.

Acknowledgements

This research has been partially supported by ANCS grant D11-025/15.09.2007. Romanian Military Topographic Department is highly acknowledged for the permission to present the quasi-


Ioannis K Neonakis

Demetrios A Spandidos

Efthimia Petinaki

Confronting multidrug-resistant Acinetobacter baumannii: a review

Keywords:

Multidrug-resistant Acinetobacter baumannii (MDR-AB) infections are difficult to treat owing to the extremely limited armamentarium. The present review reports all available treatment options against MDR-AB, including single molecules, combination schemes, and alternative modes of antimicrobial administration.

Additionally, a group of recently reported peptides with anti-MDR-AB activity is described.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A c c e p t e d M a n u s c r i p t [START_REF] Giamarellou | Acinetobacter baumannii: a universal threat to public health?[END_REF] E-mail address: ineonakis@gmail.com (I.K. Neonakis).

A c c e p t e d M a n u s c r i p t

Introduction

Over the last 10 years, Acinetobacter baumannii has emerged as one of the most problematic pathogens as treatment has been limited to only a few antibiotics. A wide spectrum of antimicrobial resistance mechanisms is exhibited by A. baumannii. Apart from its intrinsic resistance mainly due to the low permeability of the outer membrane to certain antibiotics as well as constitutive expression of certain efflux pumps, A.

baumannii is able to easily acquire and incorporate genetic elements such as plasmids, transposons and integrons [START_REF] Vila | Therapeutic options for Acinetobacter baumannii infections[END_REF][START_REF] Giamarellou | Acinetobacter baumannii: a universal threat to public health?[END_REF]. Multidrug-resistant (MDR) isolates (i.e.

those with non-susceptibility to three or more classes of drugs) are being increasingly reported worldwide [START_REF] Fournier | The epidemiology and control of Acinetobacter baumannii in health care facilities[END_REF][START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF][START_REF] Maviglia | Role of old antibiotics in multidrug resistant bacterial infections[END_REF][START_REF] Paterson | The epidemiological profile of infections with multidrug-resistant Pseudomonas aeruginosa and Acinetobacter species[END_REF][START_REF] Ko | High rates of resistance to colistin and polymyxin B in subgroups of Acinetobacter baumannii isolates from Korea[END_REF][START_REF] Doi | Extensively drugresistant Acinetobacter baumannii[END_REF]. Infections due to such resistant microbes are associated with increased morbidity and mortality [START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF][START_REF] Paterson | The epidemiological profile of infections with multidrug-resistant Pseudomonas aeruginosa and Acinetobacter species[END_REF][START_REF] Fagon | Nosocomial pneumonia in ventilated patients: a cohort study evaluating attributable mortality and hospital stay[END_REF][START_REF] Rello | Impact of previous antimicrobial therapy on the etiology and outcome of ventilator-associated pneumonia[END_REF][START_REF] Peleg | Acinetobacter baumannii: emergence of a successful pathogen[END_REF]. Carbapenems, which used to be the antimicrobials of choice, have been increasingly compromised and no longer constitute salvage therapy for A. baumannii infections [START_REF] Vila | Therapeutic options for Acinetobacter baumannii infections[END_REF][START_REF] Mcdonald | Trends in antimicrobial resistance in health care-associated pathogens and effect on treatment[END_REF][START_REF] Bassetti | Drug treatment for multidrug-resistant Acinetobacter baumannii infections[END_REF][START_REF] Falagas | Trends in antimicrobial resistance of Acinetobacter baumannii clinical isolates from hospitalised patients in Greece and treatment implications[END_REF]. The present review aimed to report all available therapeutic options against the MDR A. baumannii (MDR-AB) isolates and to present all of the latest developments in the field.

Polymyxins

Polymyxins are polycationic lipopolypeptide antimicrobials that have been available on the market for more than 50 years. They act both on the outer and cytoplasmic membranes leading to loss of membrane integrity. Among them, polymyxin B and polymyxin E [colistin (COL)] are suitable for clinical use. For a long period polymyxins were used infrequently owing to concerns of renal toxicity. However, over the last 5 years the emergence of MDR Gram-negative bacteria and the lack of new A c c e p t e d M a n u s c r i p t [START_REF] Maviglia | Role of old antibiotics in multidrug resistant bacterial infections[END_REF] antimicrobials have led to a revival of polymyxins, especially COL. There are two forms of COL: colistin methanesulfonate (CMS) for intravenous (i.v.) or intramuscular administration; and colistin sulphate for topical skin use (powder) or per os administration. Both COL forms can be inhaled. Clinical use of polymyxins against A. baumannii isolates proved to be extremely successful. An 87% cure rate was reported in a retrospective cohort analysis of patients receiving i.v. COL for microbiologically documented infections, whereas in a large retrospective study a 75% cure rate of ventilator-associated pneumonia (VAP) caused by pandrug-resistant (PDR) A. baumannii and Pseudomonas aeruginosa isolates susceptible only to COL was reported [START_REF] Falagas | Colistin therapy for microbiologically documented multidrug-resistant Gram-negative bacterial infections: a retrospective cohort study of 258 patients[END_REF][START_REF] Kallel | Safety and efficacy of colistin compared with imipenem in the treatment of ventilatorassociated pneumonia: a matched case-control study[END_REF]. Similar results have also been reported by other authors [START_REF] Holloway | Polymyxin B and doxycycline use in patients with multidrug-resistant Acinetobacter baumannii infections in the intensive care unit[END_REF][18][START_REF] Levin | Intravenous colistin as therapy for nosocomial infections caused by multidrugresistant Pseudomonas aeruginosa and Acinetobacter baumannii[END_REF].

Polymyxins have been tested extensively in combination with other agents against MDR-AB. In vitro they have been combined with carbapenems, cefepime, amikacin, azithromycin, minocycline and other antimicrobials, with promising results [START_REF] Timurkaynak | In vitro activities of non-traditional antimicrobials alone or in combination against multidrug-resistant strains of Pseudomonas aeruginosa and Acinetobacter baumannii isolated from intensive care units[END_REF][START_REF] Tan | In vitro effect of minocycline and colistin combinations on imipenem-resistant Acinetobacter baumannii clinical isolates[END_REF][START_REF] Rahal | Novel antibiotic combinations against infections with almost completely resistant Pseudomonas aeruginosa and Acinetobacter species[END_REF][START_REF] Yoon | In vitro double and triple synergistic activities of polymyxin B, imipenem, and rifampin against multidrugresistant Acinetobacter baumannii[END_REF][START_REF] Pankuch | Activity of meropenem with and without ciprofloxacin and colistin against Pseudomonas aeruginosa and Acinetobacter baumannii[END_REF].

Clinically, the combination of COL with meropenem (MEM) appears to be superior to the other agents. Falagas et al. [START_REF] Falagas | Colistin therapy for microbiologically documented multidrug-resistant Gram-negative bacterial infections: a retrospective cohort study of 258 patients[END_REF] reported that patients who were treated with a COL+MEM combination had a better outcome of infection than patients who received COL in combination with other antimicrobials. The authors further noted that although no statistically significant difference in clinical outcome was found between COL monotherapy and COL+MEM combination therapy, use of the combination is preferable since heteroresistance may arise when COL is used alone [START_REF] Falagas | Colistin therapy for microbiologically documented multidrug-resistant Gram-negative bacterial infections: a retrospective cohort study of 258 patients[END_REF]. Another clinically important combination of COL is that with rifampicin (RIF), which will be described in detail later.

A c c e p t e d M a n u s c r i p t [START_REF] Paterson | The epidemiological profile of infections with multidrug-resistant Pseudomonas aeruginosa and Acinetobacter species[END_REF] Following is a detailed examination of three issues regarding the treatment of MDR-AB with COL.

In vitro susceptibility testing

With regard to vitro susceptibility testing of COL, there are three points to be considered. The predictive accuracy of the disk diffusion test is limited owing to the poor agar diffusion characteristics of COL [START_REF] Galani | Colistin susceptibility testing by Etest and disk diffusion methods[END_REF]. Hence, the disk diffusion test is reported to be unreliable for detecting COL resistance compared with the gold standard method of agar dilution [START_REF] Galani | Colistin susceptibility testing by Etest and disk diffusion methods[END_REF][START_REF] Tan | Comparison of three standardized disc susceptibility testing methods for colistin[END_REF][START_REF] Lo-Ten-Foe | Comparative evaluation of the VITEK 2, disk diffusion, Etest, broth microdilution, and agar dilution susceptibility testing methods for colistin in clinical isolates, including heteroresistant Enterobacter cloacae and Acinetobacter baumannii strains[END_REF][START_REF] Giamarellou | Multidrug-resistant Gram-negative infections: what are the treatment options?[END_REF]. Another point of concern is that the minimum inhibitory concentrations (MICs) of COL determined by the VITEK ® 2 system (bioMérieux, Marcy l'Etoile, France), a widely used automated susceptibility testing system, have been reported to be unreliable [START_REF] Tan | Comparison of Etest, Vitek and agar dilution for susceptibility testing of colistin[END_REF]. However, this matter is controversial since contrary results have been obtained by other researchers [START_REF] Lo-Ten-Foe | Comparative evaluation of the VITEK 2, disk diffusion, Etest, broth microdilution, and agar dilution susceptibility testing methods for colistin in clinical isolates, including heteroresistant Enterobacter cloacae and Acinetobacter baumannii strains[END_REF].

Finally, it should be noted that the currently available breakpoints for COL are based on colistin sulfate and not CMS that is used intravenously [START_REF] Lo-Ten-Foe | Comparative evaluation of the VITEK 2, disk diffusion, Etest, broth microdilution, and agar dilution susceptibility testing methods for colistin in clinical isolates, including heteroresistant Enterobacter cloacae and Acinetobacter baumannii strains[END_REF].

Resistance: heteroresistance

The mechanisms of resistance to COL have yet to be adequately clarified. However, resistance has been associated with alterations in the outer layer of the bacterial cell, such as a reduction in lipopolysaccharides, reduced levels of specific outer membrane proteins, reduction of lipid content and reduction in cell envelope Mg +2 and Ca +2 contents [START_REF] Maviglia | Role of old antibiotics in multidrug resistant bacterial infections[END_REF][START_REF] Mortensen | Effects of colistin on surface ultrastructure and nanomechanics of Pseudomonas aeruginosa cells[END_REF][START_REF] Young | Role of Pseudomonas aeruginosa outer membrane protein OprH in polymyxin and gentamicin resistance: isolation of an OprH-deficient mutant by gene replacement techniques[END_REF]. Furthermore, efflux of potassium has been correlated with resistance to polymyxin B in Yersinia spp. [START_REF] Bengoechea | Temperature-regulated efflux pump/potassium antiporter system mediates resistance to cationic antimicrobial peptides in Yersinia[END_REF].

A c c e p t e d M a n u s c r i p t [START_REF] Ko | High rates of resistance to colistin and polymyxin B in subgroups of Acinetobacter baumannii isolates from Korea[END_REF] Resistance to polymyxins is still considered to be rare. However, a number of sporadic cases of infections caused by COL-resistant isolates have been reported [START_REF] Falagas | Pandrug-resistant Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii infections: characteristics and outcome in a series of 28 patients[END_REF][START_REF] Giamarellou | Colistin: the loss of the last frontier?[END_REF][START_REF] Beno | Bacteraemia in cancer patients caused by colistin-resistant Gram-negative bacilli after previous exposure to ciprofloxacin and/or colistin[END_REF][START_REF] Souli | Emergence of extensively drug-resistant and pandrug-resistant Gram-negative bacilli in Europe[END_REF]. Moreover, Gales et al. [START_REF] Gales | Global assessment of the antimicrobial activity of polymyxin B against 54 731 clinical isolates of Gram-negative bacilli: report from the SENTRY antimicrobial surveillance programme (2001-2004)[END_REF] reported 2.7% and 1.7% polymyxin B-resistant A. baumannii isolates collected in Europe and in North and Latin America, respectively. Souli et al. [START_REF] Souli | In vitro activity of tigecycline against multiple-drugresistant, including pan-resistant, Gram-negative and Gram-positive clinical isolates from Greek hospitals[END_REF] reported a 3% COL resistance rate among 100 A. baumannii strains derived from Intensive Care Unit patients in Greece. Notably, Ko et al. [START_REF] Ko | High rates of resistance to colistin and polymyxin B in subgroups of Acinetobacter baumannii isolates from Korea[END_REF] reported extremely high resistance rates (27.9%) among 214 isolates of A. baumannii from two South Korean hospitals.

For the first time in 2006, Li et al. [START_REF] Li | Heteroresistance to colistin in multidrug-resistant Acinetobacter baumannii[END_REF] reported the phenomenon of heteroresistance to COL in MDR-AB clinical isolates. Using population analysis profiles (PAPs) of 16 clinical isolates with MICs of COL in the range 0.25-2 g/mL, the authors detected the presence of subpopulations (<0.1% of inocula) that were able to grow in the presence of COL at 3-10 g/mL. The resistant subpopulations were not detected by either the disk diffusion susceptibility test or by commercial automated systems.

Owing to resistant subpopulations, significant re-growth was observed at 24 h following the rapid initial killing (<2 h) in time-kill studies. Notably, heteroresistance occurred in almost all the isolates (15/16) [START_REF] Li | Heteroresistance to colistin in multidrug-resistant Acinetobacter baumannii[END_REF]. Hawley et al. [40] found a similar high percentage of heteroresistance (100%), whereas in a more recent study by Yau et al. [START_REF] Yau | Colistin hetero-resistance in multidrug-resistant Acinetobacter baumannii clinical isolates from the Western Pacific region in the SENTRY antimicrobial surveillance programme[END_REF] the percentage was determined to be 23%. Li et al. [START_REF] Li | Heteroresistance to colistin in multidrug-resistant Acinetobacter baumannii[END_REF] found that in the majority of clinical isolates a subpopulation of 0.00001-0.000001% grew in the presence of 10 g/mL COL. This growth indicates that the COL plasma concentrations of 0.5-3.5 g/mL usually achieved with the recommended dosage regimens of CMS may be A c c e p t e d M a n u s c r i p t [START_REF] Doi | Extensively drugresistant Acinetobacter baumannii[END_REF] unable to eradicate the more resistant subpopulations [START_REF] Giamarellou | Multidrug-resistant Gram-negative infections: what are the treatment options?[END_REF][42][START_REF] Li | Steadystate pharmacokinetics of intravenous colistin methanesulphonate in patients with cystic fibrosis[END_REF][START_REF] Li | Pharmacokinetics of colistin methanesulfonate and colistin in a critically ill patient receiving continuous venovenous hemodiafiltration[END_REF]. Hawley et al. [40] found that the degree of heteroresistance among isolates from patients with previous COL treatment was statistically significantly higher than that of isolates not previously exposed to COL. Furthermore, Li et al. [START_REF] Li | Heteroresistance to colistin in multidrug-resistant Acinetobacter baumannii[END_REF] reported that following exposure to COL a substantial increase in the proportion of resistant subpopulations was noted. These findings suggest that in order to avoid suboptimal concentrations of COL, further pharmacodynamic investigations on dosage optimisation of CMS are required and that CMS monotherapy should be avoided. In addition, it has been proposed that mini-PAPs with COL at >2 mg/L should be employed for susceptibility reporting in clinical microbiological laboratories [START_REF] Yau | Colistin hetero-resistance in multidrug-resistant Acinetobacter baumannii clinical isolates from the Western Pacific region in the SENTRY antimicrobial surveillance programme[END_REF].

Side effects

The most common potential toxicities of i.v. CMS administration are nephrotoxicity and neurotoxicity, with the latter being relatively rare [START_REF] Nation | Colistin in the 21st century[END_REF]. A high incidence of nephrotoxicity was reported in early studies [START_REF] Wolinsky | Neurotoxic and nephrotoxic effects of colistin in patients with renal disease[END_REF][START_REF] Koch-Weser | Adverse effects of sodium colistimethate. Manifestations and specific reaction rates during 317 courses of therapy[END_REF]. However, a series of recent investigations revealed that COL is not as nephrotoxic as initially believed [48][START_REF] Falagas | Toxicity after prolonged (more than four weeks) administration of intravenous colistin[END_REF][START_REF] Kasiakou | Combination therapy with intravenous colistin for management of infections due to multidrug-resistant Gram-negative bacteria in patients without cystic fibrosis[END_REF][START_REF] Reina | Safety and efficacy of colistin in Acinetobacter and Pseudomonas infections: a prospective cohort study[END_REF].

Falagas et al. [START_REF] Falagas | Toxicity after prolonged (more than four weeks) administration of intravenous colistin[END_REF] analysed data for 19 courses of prolonged i.v. COL administration (mean ± standard deviation duration of administration 43.4 ± 14.6 days). The median creatinine value increased by 0.25 mg/dL during treatment compared with the baseline, but returned close to the baseline at the end of treatment [START_REF] Falagas | Toxicity after prolonged (more than four weeks) administration of intravenous colistin[END_REF]. Stein and Raoult [START_REF] Stein | Colistin: an antimicrobial for the 21st century?[END_REF] reported that a dose of 1 million units of CMS every 8 h was effective and safe, even when administered over a period of 3-6 months for infections associated with orthopaedic devices. In another study, i.v. administration of CMS (5.0 mg/kg/24 h in three doses) was generally well tolerated by critically ill patients with normal A c c e p t e d M a n u s c r i p t [START_REF] Fagon | Nosocomial pneumonia in ventilated patients: a cohort study evaluating attributable mortality and hospital stay[END_REF] renal function in whom it was used as salvage therapy [START_REF] Reina | Safety and efficacy of colistin in Acinetobacter and Pseudomonas infections: a prospective cohort study[END_REF]. Nevertheless, prolonged administration of CMS increases the risk of nephrotoxicity [START_REF] Hartzell | Nephrotoxicity associated with intravenous colistin (colistimethate sodium) treatment at a tertiary care medical center[END_REF]. This risk does not appear to be related to the daily dose of CMS but rather the total cumulative dose [START_REF] Nation | Colistin in the 21st century[END_REF][START_REF] Hartzell | Nephrotoxicity associated with intravenous colistin (colistimethate sodium) treatment at a tertiary care medical center[END_REF]. Subsequently, careful monitoring of kidney function in patients requiring prolonged therapy with CMS is crucial along with efforts to avoid prolonged treatment courses, whenever possible.

Tigecycline (TIG)

TIG is a semisynthetic glycylcycline. It represents a modified minocycline and has activity against a broad spectrum of Gram-positive (including meticillin-resistant staphylococci and vancomycin-resistant enterococci), Gram-negative (including extended-spectrum -lactamase-and Amp C-producing Enterobacteriaceae) and anaerobic organisms (including Bacteroides spp. and Clostridium perfringens). TIG inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit [START_REF] Bergeron | Glycylcyclines bind to the high-affinity tetracycline ribosomal binding site and evade Tet(M)-and Tet(O)-mediated ribosomal protection[END_REF]. A 100 mg loading dose is recommended, followed by 50 mg i.v. every 12 h. Biliary or faecal excretion accounts for 59% of the administered dose [START_REF] Kasbekar | Tigecycline: a new glycylcycline antimicrobial agent[END_REF][START_REF] Bhattacharya | Tigecycline[END_REF].

A serious issue regarding TIG is that the disk diffusion technique, when employed either for susceptibility testing or for MIC determination by Etest, may lead to serious discrepancies in the results compared with results by the microdilution method [START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF][START_REF] Casal | Influence of testing methodology on the tigecycline activity profile against presumably tigecycline-non-susceptible Acinetobacter spp[END_REF][START_REF] Canigia | Susceptibility testing of tigecycline against Acinetobacter spp. by disc diffusion method: withdrawing a therapeutic option by varying the Mueller-Hinton agar?[END_REF]. The quality of the Mueller-Hinton media plays an important role. TIG activity is affected by the cation concentration of the Mueller-Hinton agar and, moreover, TIG suffers degradation by an oxidation process, leading to loss of A c c e p t e d M a n u s c r i p t [START_REF] Rello | Impact of previous antimicrobial therapy on the etiology and outcome of ventilator-associated pneumonia[END_REF] potency [START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF]. Thus, for correct evaluation of susceptibility to TIG, the Mueller-Hinton agar must be properly and freshly prepared or anaerobically stored [START_REF] Hope | Effect of medium type, age and aeration on the MICs of tigecycline and classical tetracyclines[END_REF][START_REF] Petersen | Effect of medium age and supplementation with the biocatalytic oxygen-reducing reagent oxyrase on in vitro activities of tigecycline against recent clinical isolates[END_REF].

Despite its favourable activity against Gram-positive microorganisms, TIG is mainly reserved for use against MDR Gram-negative bacteria owing to the lack of therapeutic alternatives against these bacteria. Its use against A. baumannii is offlabel. Moreover, no defined susceptibility breakpoints have been established thus far for A. baumannii. Even in their latest editions (2010), neither the Clinical and Laboratory Standards Institute (CLSI) [START_REF]Performance standards for antimicrobial susceptibility testing; twentieth informational supplement[END_REF] nor the British Society for Antimicrobial Chemotherapy (BSAC) [START_REF]BSAC methods for antimicrobial susceptibility testing[END_REF] propose any specific breakpoints for TIG regarding A. baumannii owing to insufficient clinical evidence. However, the limited armamentarium against A. baumannii and the relatively low MICs obtained in various studies rendered the use of TIG against A. baumannii appealing. In 2007 in a major global study, Reinert et al. [START_REF] Reinert | Antimicrobial susceptibility among organisms from the Asia/Pacific Rim, Europe and Latin and[END_REF] reported MIC 90 values (MICs for 90% of the isolates) of 1 mg/L for Europe, North America, Latin America and the Asia/Pacific Rim.

Recently, using a global collection of 6292 isolates, Garrison et al. [START_REF] Garrison | In vitro activity of tigecycline and comparator agents against a global collection of Gram-negative and Grampositive organisms: Tigecycline Evaluation and Surveillance Trial 2004 to 2007[END_REF] reported an MIC 50 (MIC for 50% of the isolates) at 0.5 mg/L and an MIC 90 at 1 mg/L, with an MIC range of ≤0.008-16 mg/L. Furthermore, when testing only the MDR-AB subset (n = 582), the authors reported an MIC 50 at 1 mg/L and an MIC 90 at 2 mg/L, with an MIC range of ≤0.008-8 mg/L [START_REF] Garrison | In vitro activity of tigecycline and comparator agents against a global collection of Gram-negative and Grampositive organisms: Tigecycline Evaluation and Surveillance Trial 2004 to 2007[END_REF]. Similar in vitro results were also reported by Draghi et Whilst TIG retains good in vitro activity against MDR-AB, its clinical efficacy remains a matter of controversy. The efficacy of TIG in 17 patients with selected serious A c c e p t e d M a n u s c r i p t [START_REF] Peleg | Acinetobacter baumannii: emergence of a successful pathogen[END_REF] infections caused by resistant bacteria was evaluated in a phase III, open-label, noncomparative, multicentre study [START_REF] Vasilev | A Phase 3, open-label, non-comparative study of tigecycline in the treatment of patients with selected serious infections due to resistant Gram-negative organisms including Enterobacter species, Acinetobacter baumannii and Klebsiella pneumoniae[END_REF]. The clinical cure rate with TIG therapy observed in microbiologically evaluable patients was 82.4% and the microbiological eradication rate was 64.7% [START_REF] Vasilev | A Phase 3, open-label, non-comparative study of tigecycline in the treatment of patients with selected serious infections due to resistant Gram-negative organisms including Enterobacter species, Acinetobacter baumannii and Klebsiella pneumoniae[END_REF]. Gordon and Wareham [START_REF] Gordon | A review of clinical and microbiological outcomes following treatment of infections involving multidrug-resistant Acinetobacter baumannii with tigecycline[END_REF] reported an overall 68% positive clinical outcome in 34 patients with infections involving MDR-AB. Karageorgopoulos et al. [START_REF] Karageorgopoulos | Tigecycline for the treatment of multidrug-resistant (including carbapenem-resistant)[END_REF] reviewed the effectiveness of TIG for MDR-AB infections in eight identified clinical studies, reporting retrospective data regarding 42 severely ill patients, 31 of whom had respiratory tract infection (in 4 cases with secondary bacteraemia) and 4 had bacteraemia. TIG therapy (in combination with other antibiotics in 28 patients) was effective in 32/42 cases. Poulakou et al. [START_REF] Poulakou | Tigecycline in the treatment of infections from multi-drug resistant Gramnegative pathogens[END_REF] retrospectively analysed adult patients who had received TIG for >5 days either as monotherapy (M group) or as presumed active monotherapy (PAM group). In the PAM group, all co-administered antimicrobial(s) were resistant in vitro against the targeted pathogen(s) or failed clinically and microbiologically after ≥ 5 days of therapy despite in vitro susceptibility [START_REF] Poulakou | Tigecycline in the treatment of infections from multi-drug resistant Gramnegative pathogens[END_REF]. Among the 15 cases of MDR-AB isolation from patients in the M group, the clinical response was characterised as improvement or cure in 11 cases. Among the 13 cases of MDR-AB isolation from patients in the PAM group, the clinical response was characterised as improvement/cure in 10 cases, leading to an overall 75% (21/28) successful clinical response rate regarding the 28 A. baumannii isolates. In contrast to the aforementioned studies, Gallagher and Rouse [START_REF] Gallagher | Tigecycline for the treatment of Acinetobacter infections: a case series[END_REF] reported poor favourable clinical outcome (improvement/cure) with TIG therapy in MDR-AB infections (29%; 8/28 patients). Similarly, a low rate of favourable clinical outcome was reported by Anthony et al. [START_REF] Anthony | Clinical and microbiological outcomes of serious infections with multidrugresistant Gram-negative organisms treated with tigecycline[END_REF]. The authors reported that four of five patients with serious infections by MDR-AB characterised as intermediately resistant to TIG (MIC >2 g/mL or <8 g/mL) succumbed (deaths were related to infection), whereas A c c e p t e d M a n u s c r i p t [START_REF] Mcdonald | Trends in antimicrobial resistance in health care-associated pathogens and effect on treatment[END_REF] 0 of 4 patients with pre-therapy isolates susceptible to TIG (MIC ≤ 2 g/mL) succumbed (P = 0.048) [START_REF] Anthony | Clinical and microbiological outcomes of serious infections with multidrugresistant Gram-negative organisms treated with tigecycline[END_REF]. More information on the clinical efficacy of TIG therapy from well designed, prospective studies using a large number of patients is crucial.

In the latter aforementioned study by Anthony et al. [START_REF] Anthony | Clinical and microbiological outcomes of serious infections with multidrugresistant Gram-negative organisms treated with tigecycline[END_REF] it was also reported that one patient with an A. baumannii tracheal aspirate isolate that was initially susceptible developed resistance during therapy (MIC increased from 2.00 g/mL to 12.00 g/mL after 14 days). The difficulty arising with the emergence of resistance among MDR Gram-negative organisms during therapy with TIG has previously been described.

Schafer et al. [START_REF] Schafer | Early experience with tigecycline for ventilator-associated pneumonia and bacteremia caused by multidrug-resistant Acinetobacter baumannii[END_REF] reported that one patient with VAP plus bacteraemia due to MDR-AB developed resistance to TIG during therapy. A similar case was reported by Reid et al. [START_REF] Reid | Rapid development of Acinetobacter baumannii resistance to tigecycline[END_REF] who concluded that clinicians should be aware that TIG MICs for A. baumannii isolates may increase during therapy with TIG following only brief exposure to the drug. Patients receiving TIG for Acinetobacter should be monitored for the development of clinical resistance and isolates should be continuously monitored [START_REF] Reid | Rapid development of Acinetobacter baumannii resistance to tigecycline[END_REF].

Superinfections with pathogens inherently resistant to TIG (P. aeruginosa, Proteus spp., Providencia spp., etc.) are a matter of great concern. A superinfection is defined as a new isolate that emerges during therapy at the original site of infection, with the emergence or worsening of clinical signs and symptoms of infection [START_REF] Vasilev | A Phase 3, open-label, non-comparative study of tigecycline in the treatment of patients with selected serious infections due to resistant Gram-negative organisms including Enterobacter species, Acinetobacter baumannii and Klebsiella pneumoniae[END_REF].

Poulakou et al. [START_REF] Poulakou | Tigecycline in the treatment of infections from multi-drug resistant Gramnegative pathogens[END_REF] reported that among 45 patients treated with TIG for MDR or PDR infections, 10 episodes of superinfections by inherently resistant pathogens (9 bacteraemias and 1 urinary tract infection) occurred. The possibility of A c c e p t e d M a n u s c r i p t [START_REF] Bassetti | Drug treatment for multidrug-resistant Acinetobacter baumannii infections[END_REF] superinfections should be seriously considered, and TIG monotherapy should be avoided when the risk for pseudomonal infection is significant.

Owing to the low mean peak serum concentrations of TIG achievable at recommended doses, therapeutic failure of A. baumannii bacteraemia is plausible, even for strains with low MIC values [START_REF] Anthony | Clinical and microbiological outcomes of serious infections with multidrugresistant Gram-negative organisms treated with tigecycline[END_REF][START_REF] Peleg | Acinetobacter baumannii bloodstream infection while receiving tigecycline: a cautionary report[END_REF]. Consequently, TIG treatment should generally be avoided in cases of bacteraemia due to A. baumannii, especially for strains with MICs ≥ 1 g/mL. In addition, clinicians should avoid the use of TIG for infections in poorly penetrated anatomical sites such as the urinary tract because therapeutic failure is feasible and because such treatments may promote the development of further resistance [START_REF] Anthony | Clinical and microbiological outcomes of serious infections with multidrugresistant Gram-negative organisms treated with tigecycline[END_REF].

TIG has been investigated in combination with other antibacterials against A. baumannii isolates. Enhanced activity or bactericidal synergism occurred with TIG plus carbapenems, levofloxacin, amikacin and RIF in various in vitro studies [START_REF] Principe | In vitro activity of tigecycline in combination with various antimicrobials against multidrug resistant Acinetobacter baumannii[END_REF][77][START_REF] Lim | In vitro activity of various combinations of antimicrobials against carbapenem-resistant Acinetobacter species in Singapore[END_REF].

Results of the combination of TIG with COL are controversial [START_REF] Petersen | In vitro antibacterial activities of tigecycline in combination with other antimicrobial agents determined by chequerboard and time-kill kinetic analysis[END_REF][START_REF] Arroyo | In vitro activities of tigecycline, minocycline, and colistin-tigecycline combination against multi-and pandrugresistant clinical isolates of Acinetobacter baumannii group[END_REF][START_REF] Dizbay | In vitro synergistic activity of tigecycline and colistin against XDR-Acinetobacter baumannii[END_REF].

Apart from the in vitro studies, there are reports of single cases of MDR-AB infections successfully treated with combinations of TIG with other agents such as MEM, COL, piperacillin/tazobactam and trimethoprim/sulfamethoxazole [77, [START_REF] Leclerc | Treatment of a septic shock due to multiresistant Acinetobacter baumannii with tigecycline in combination [in French][END_REF][START_REF] Taccone | Successful treatment of septic shock due to pan-resistant Acinetobacter baumannii using combined antimicrobial therapy including tigecycline[END_REF]. In a retrospective case series, Schafer et al. [START_REF] Schafer | Early experience with tigecycline for ventilator-associated pneumonia and bacteremia caused by multidrug-resistant Acinetobacter baumannii[END_REF] reported clinical cure in 9/9 patients (100%) treated with TIG + imipenem (IPM), in 4/7 patients (57%) treated with TIG+COL and in 3/4 patients (75%) treated with a combination of the three drugs.

However, the potential clinical role of TIG in combination with other agents against A. baumannii infections should be further investigated.

Sulbactam

Sulbactam is an Ambler class A -lactamase inhibitor with a structure similar to lactams that carries intrinsic activity against A. baumannii by binding to its penicillinbinding protein 2 [START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF]. A series of studies have reported encouraging results on the efficacy of sulbactam against A. baumannii infections and it is generally accepted that in the cases of sulbactam-susceptible A. baumannii infections, sulbactam should be considered as the preferred therapeutic solution [START_REF] Peleg | Acinetobacter baumannii: emergence of a successful pathogen[END_REF][START_REF] Choi | Comparison of efficacy of cefoperazone/sulbactam and imipenem/cilastatin for treatment of Acinetobacter bacteremia[END_REF][START_REF] Cisneros | Bacteremia due to Acinetobacter baumannii: epidemiology, clinical findings, and prognostic features[END_REF][START_REF] Jellison | Epidemiology, resistance, and outcomes of Acinetobacter baumannii bacteremia treated with imipenemcilastatin or ampicillin-sulbactam[END_REF][START_REF] Peleg | Optimizing therapy for Acinetobacter baumannii[END_REF][START_REF] Smolyakov | Nosocomial multi-drug resistant Acinetobacter baumannii bloodstream infection: risk factors and outcome with ampicillin-sulbactam treatment[END_REF][START_REF] Levin | Severe nosocomial infections with imipenem-resistant Acinetobacter baumannii treated with ampicillin/sulbactam[END_REF][START_REF] Lode | Rational antibiotic therapy and the position of ampicillin/sulbactam[END_REF][START_REF] Corbella | Efficacy of sulbactam alone and in combination with ampicillin in nosocomial infections caused by multiresistant Acinetobacter baumannii[END_REF][92]. Even in the case of carbapenem-resistant A. baumannii infections, sulbactam appears to be more efficacious than polymyxins [START_REF] Oliveira | Ampicillin/sulbactam compared with polymyxins for the treatment of infections caused by carbapenem-resistant Acinetobacter spp[END_REF]. Moreover, it has been shown that the pharmaceutical cost is significantly reduced when sulbactam is selected instead of other antibiotics such as IPM [START_REF] Jellison | Epidemiology, resistance, and outcomes of Acinetobacter baumannii bacteremia treated with imipenemcilastatin or ampicillin-sulbactam[END_REF]. Although more data on optimal dosing and mode of administration are required, it has been proposed that 8 g of the sulbactam compound in divided doses, assuming normal renal function, should be administered per day [START_REF] Vila | Therapeutic options for Acinetobacter baumannii infections[END_REF][START_REF] Zavascki | Multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii: resistance mechanisms and implications for therapy[END_REF][START_REF] Peleg | Optimizing therapy for Acinetobacter baumannii[END_REF][START_REF] Lode | Rational antibiotic therapy and the position of ampicillin/sulbactam[END_REF].

A disadvantage is that in a number of countries sulbactam is commercially available only in combination with ampicillin at a fixed 2:1 ratio. Another difficulty involves the increasing number of sulbactam-resistant A. baumannii isolates. In a Spanish multicentre study, only 46.7% of A. baumannii isolates were susceptible to this agent [START_REF] Fernandez-Cuenca | Clonal diversity and antimicrobial susceptibility of Acinetobacter baumannii isolated in Spain. A nationwide multicenter study: GEIH-Ab project (2000)[END_REF], whereas in a major recent study from Taiwan ca. 70% of the clinical isolates were resistant to ampicillin/sulbactam (SAM) [START_REF] Yang | Prevalence of antibiotics resistance and OXA carbapenemases genes in multidrug-resistant Acinetobacter baumannii isolates in central Taiwan[END_REF].

A c c e p t e d M a n u s c r i p t [START_REF] Falagas | Colistin therapy for microbiologically documented multidrug-resistant Gram-negative bacterial infections: a retrospective cohort study of 258 patients[END_REF] Enhanced effectiveness has been found when sulbactam is combined with a series of other molecules such as cefepime, IPM, MEM, amikacin and RIF [START_REF] Peleg | Optimizing therapy for Acinetobacter baumannii[END_REF][START_REF] Tong | In vitro activity of cefepime combined with sulbactam against clinical isolates of carbapenem-resistant Acinetobacter spp[END_REF][START_REF] Song | In vitro activities of carbapenem/sulbactam combination, colistin, colistin/rifampicin combination and tigecycline against carbapenem-resistant Acinetobacter baumannii[END_REF][START_REF] Savov | In vitro investigation of the susceptibility of Acinetobacter baumannii strains isolated from clinical specimens[END_REF][START_REF] Ko | In vitro and in vivo activity of meropenem and sulbactam against a multidrug-resistant Acinetobacter baumannii strain[END_REF][START_REF] Appleman | In vitro activities of nontraditional antimicrobials against multiresistant Acinetobacter baumannii strains isolated in an intensive care unit outbreak[END_REF][START_REF] Kiffer | In vitro synergy test of meropenem and sulbactam against clinical isolates of Acinetobacter baumannii[END_REF].

However, these are in vitro studies or studies on animal models without clinical evaluation in humans. More data are therefore imperative to confirm this observation.

Rifampicin

RIF is generally considered as an antimicrobial best suited for Gram-positive bacteria. However, in vitro studies and experimental models of infection showed that RIF alone could demonstrate a bactericidal effect on MDR-AB [START_REF] Xirouchaki | Interactions of colistin and rifampin on multidrug-resistant Acinetobacter baumannii[END_REF][START_REF] Wolff | In vivo efficacies of combinations of -lactams, -lactamase inhibitors, and rifampin against Acinetobacter baumannii in a mouse pneumonia model[END_REF][START_REF] Montero | Efficacy of colistin versus -lactams, aminoglycosides, and rifampin as monotherapy in a mouse model of pneumonia caused by multiresistant Acinetobacter baumannii[END_REF]. In these experimental models, the MICs of RIF for the A. baumannii strains used ranged from 4 mg/L to 8 mg/L, and RIF monotherapy proved to be as efficacious as that of IPM and more efficacious than that of COL monotherapy with regard to bacterial clearance from the lungs [START_REF] Wolff | In vivo efficacies of combinations of -lactams, -lactamase inhibitors, and rifampin against Acinetobacter baumannii in a mouse pneumonia model[END_REF][START_REF] Montero | Efficacy of colistin versus -lactams, aminoglycosides, and rifampin as monotherapy in a mouse model of pneumonia caused by multiresistant Acinetobacter baumannii[END_REF]. However, it is well documented that when used alone RIF shows rapid development of resistance, thus its combination with other antimicrobials is necessary [105,[START_REF] Pachon-Ibanez | Prevention of rifampicin resistance in Acinetobacter baumannii in an experimental pneumonia murine model, using rifampicin associated with imipenem or sulbactam[END_REF]. Although an additive effect or synergism have been observed for combinations of RIF with various agents such as carbapenems, TIG or SAM [START_REF] Dizbay | In vitro synergistic activity of tigecycline and colistin against XDR-Acinetobacter baumannii[END_REF]105], the most promising combination is that of RIF with COL.

Early in vitro studies by Giamarellos-Bourboulis et al. [START_REF] Xirouchaki | Interactions of colistin and rifampin on multidrug-resistant Acinetobacter baumannii[END_REF] revealed the existence of synergy between RIF and COL against MDR-AB. Furthermore, Rodriguez et al. [START_REF] Rodriguez | In vitro antimicrobials activity against endemic Acinetobacter baumannii multiresistant clones[END_REF] demonstrated that the combination of RIF+COL inhibits resistant subpopulations in heteroresistant isolates irrespective of the RIF MIC values. Apart from the in vitro studies, synergy between COL and RIF has also been shown in experimental models. Using a thigh experimental model of infection by MDR-AB, Pantopoulou et A c c e p t e d M a n u s c r i p t [START_REF] Kallel | Safety and efficacy of colistin compared with imipenem in the treatment of ventilatorassociated pneumonia: a matched case-control study[END_REF] al. [START_REF] Pantopoulou | Colistin offers prolonged survival in experimental infection by multidrug-resistant Acinetobacter baumannii: the significance of coadministration of rifampicin[END_REF] demonstrated that the activity of COL was increased significantly by the presence of RIF and that the combination was effective in prolonging the survival of infected rats. Based on these results, Bassetti et al. [START_REF] Bassetti | Colistin and rifampicin in the treatment of multidrug-resistant Acinetobacter baumannii infections[END_REF] conducted a clinical trial including 29 critically ill patients with MDR-AB infections who were treated with a combination of i.v. COL and RIF. Clinical and microbiological favourable responses were observed in 22/29 cases (76%) and the overall infection-related mortality was 21% (6/29 cases). No case developed resistance to either RIF or COL. Of the 29 patients, 3 (10%) developed nephrotoxicity (all of them had previous renal failure), whereas no cases of renal failure were observed among patients with normal baseline renal function [START_REF] Bassetti | Colistin and rifampicin in the treatment of multidrug-resistant Acinetobacter baumannii infections[END_REF]. Similar results have been reported in other clinical trials, indicating that the combination of COL+RIF may be considered as a serious therapeutic option [START_REF] Petrosillo | Combined colistin and rifampicin therapy for carbapenem-resistant Acinetobacter baumannii infections: clinical outcome and adverse events[END_REF][111][START_REF] Song | Colistin and rifampicin combination in the treatment of ventilator-associated pneumonia caused by carbapenem-resistant Acinetobacter baumannii[END_REF]. However, this conclusion should be confirmed by additional clinical trials with much larger numbers of patients. An issue that should also be clarified is the role of the initial MIC to RIF in the final outcome of combination therapy. This is a critical point, since there are data supporting the fact that in cases with an initial high level of resistance to RIF (MICs ≥ 64 mg/L) a poor microbiological eradication rate is to be expected [START_REF] Song | Colistin and rifampicin combination in the treatment of ventilator-associated pneumonia caused by carbapenem-resistant Acinetobacter baumannii[END_REF].

Minocycline and fosfomycin

Minocycline is a tetracycline derivative introduced in the 1960s. However, its use was limited due to the introduction of more potent agents. Minocycline inhibits bacterial protein synthesis and acts both against Gram-positive and Gram-negative bacteria.

Recently, owing to the shortage of therapeutic solutions, its use against minocyclinesusceptible MDR-AB isolates has gained new interest. The literature is promising A c c e p t e d M a n u s c r i p t [START_REF] Holloway | Polymyxin B and doxycycline use in patients with multidrug-resistant Acinetobacter baumannii infections in the intensive care unit[END_REF] although limited [START_REF] Bishburg | Minocycline-an old drug for a new century: emphasis on methicillin-resistant Staphylococcus aureus (MRSA) and Acinetobacter baumannii[END_REF]. In one study, 7/8 patients treated with oral minocycline for traumatic wound infections by MDR-AB yielded positive outcomes [START_REF] Griffith | Minocycline therapy for traumatic wound infections caused by the multidrug-resistant Acinetobacter baumannii-Acinetobacter calcoaceticus complex[END_REF], whereas in another report i.v. treatment with minocycline was successful in 3/4 patients with MDR-AB VAP [START_REF] Wood | Tetracyclines for treating multidrug-resistant Acinetobacter baumannii ventilator-associated pneumonia[END_REF]. Since minocycline achieves ideal blood and tissue levels and has notable central nervous system penetration, it appears to play a significant role in confronting MDR-AB infections [START_REF] Bishburg | Minocycline-an old drug for a new century: emphasis on methicillin-resistant Staphylococcus aureus (MRSA) and Acinetobacter baumannii[END_REF]. More in vitro and clinical data regarding minocycline are required to verify this role.

Fosfomycin is an old bactericidal molecule that inhibits synthesis of the bacterial cell wall peptidoglycan. It acts both against Gram-positive and Gram-negative bacteria and its toxicity is very low. Although it has been found to be a reliable solution for MDR Enterobacteriaceae [START_REF] Falagas | Fosfomycin for the treatment of multidrug-resistant, including extended-spectrum -lactamase producing, Enterobacteriaceae infections: a systematic review[END_REF], fosfomycin cannot be considered as an option for MDR-AB isolates since the majority of these isolates are not susceptible to this antimicrobial agent [START_REF] Falagas | Fosfomycin for the treatment of infections caused by multidrug-resistant non-fermenting Gramnegative bacilli: a systematic review of microbiological, animal and clinical studies[END_REF][START_REF] Falagas | Antimicrobial susceptibility of multidrugresistant Gram negative bacteria to fosfomycin[END_REF][START_REF] Dobrewski | Genotypic diversity and antibiotic susceptibility of Acinetobacter baumannii isolates in a Bulgarian hospital[END_REF][START_REF] Martinez-Martinez | In-vitro activity of antimicrobial agent combinations against multiresistant Acinetobacter baumannii[END_REF].

Continuous administration of intravenous antibiotics

To optimise the effect of available agents, alternative therapeutic approaches have been used. The most promising approach is extended or continuous administration of i.v. antibacterials. It appears that continuous i.v. infusion of antibacterials with timedependent bacterial killing is superior to normal intermittent i.v. administration [START_REF] Kasiakou | Continuous versus intermittent intravenous administration of antibacterials with time-dependent action: a systematic review of pharmacokinetic and pharmacodynamic parameters[END_REF].

Extended infusion of -lactams such as carbapenems or cefepime has the ability to achieve drug concentrations above the MIC for a greater time for less susceptible organisms, especially those with an MIC between 4 g/mL and 16 g/mL [START_REF] Fishbain | Treatment of Acinetobacter infections[END_REF][START_REF] Li | Population pharmacokinetic analysis and dosing regimen optimization of meropenem in adult patients[END_REF][START_REF] Jaruratanasirikul | Comparison of the pharmacodynamics of meropenem in patients with ventilator-associated pneumonia following administration by 3-hour infusion or bolus injection[END_REF].

In a meta-analysis of 730 episodes of infection it was shown that when the total daily A c c e p t e d M a n u s c r i p t [18] antibiotic dose was the same, clinical failure was significantly lower in the continuous infusion group compared with the intermittent infusion group [START_REF] Kasiakou | Continuous versus intermittent intravenous administration of antibiotics: a metaanalysis of randomised controlled trials[END_REF]. Furthermore, there are data suggesting that continuous antibiotic dosing offers more favourable activity against resistant pathogens and may actually reduce the incidence of antibiotic resistance [START_REF] Vergidis | New antibiotic agents for bloodstream infections[END_REF]. However, more data regarding A. baumannii are necessary before firm conclusions can be drawn.

New agents: peptides

A series of new molecules, including natural products and newly synthesised chemical substances, are constantly being screened for their activity against A. baumannii, with varying results [START_REF] Barbuceanu | Synthesis, characterization and evaluation of antibacterial activity of some thiazolo[3,2-b][1,2,4]triazole incorporating diphenylsulfone moieties[END_REF][START_REF] Almajan | Synthesis, characterization and antibacterial activity of some triazole Mannich bases carrying diphenylsulfone moieties[END_REF][START_REF] Duraipandiyan | Antibacterial and antifungal activity of flindersine isolated from the traditional medicinal plant, Toddalia asiatica (L.) Lam[END_REF][START_REF] Tan | The antibacterial properties of Malaysian tualang honey against wound and enteric microorganisms in comparison to manuka honey[END_REF][START_REF] Aytemir | A study of cytotoxicity of novel chlorokojic acid derivatives with their antimicrobial and antiviral activities[END_REF][START_REF] Schwan | Screening a mushroom extract library for activity against Acinetobacter baumannii A c c e p t e d M a n u s c r i p t[END_REF]. The most notable and promising group of new substances is that of the peptides reviewed below [START_REF] Conlon | The alyteserins: two families of antimicrobial peptides from the skin secretions of the midwife toad Alytes obstetricans (Alytidae)[END_REF][START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF][START_REF] Powers | The relationship between peptide structure and antibacterial activity[END_REF][START_REF] Saugar | Activity of cecropin A-melittin hybrid peptides against colistin-resistant clinical strains of Acinetobacter baumannii: molecular basis for the differential mechanisms of action[END_REF].

Peptides normally comprise between 10 and 48 amino acid residues with no conserved domains associated with biological activity [START_REF] Conlon | The alyteserins: two families of antimicrobial peptides from the skin secretions of the midwife toad Alytes obstetricans (Alytidae)[END_REF]. With a few exceptions, peptides are cationic and contain 50% hydrophobic amino acids [START_REF] Conlon | The alyteserins: two families of antimicrobial peptides from the skin secretions of the midwife toad Alytes obstetricans (Alytidae)[END_REF]. Peptides with antimicrobial activities have been isolated from a wide range of bacterial species, plants and animals [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF]. They provide a host defence system to combat infections and play one of the most important roles against pathogenic microorganisms [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF]. They bind the negatively charged parts of the bacterial membrane by electrostatic interactions and it is postulated that they exert their inhibitory effects by increasing bacterial membrane permeability, causing leakage of bacterial contents and consequently maximal entry of several other substrates [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF].

However, a major obstacle to the development of peptide-based anti-infective drugs, A c c e p t e d M a n u s c r i p t [START_REF] Levin | Intravenous colistin as therapy for nosocomial infections caused by multidrugresistant Pseudomonas aeruginosa and Acinetobacter baumannii[END_REF] particularly if they are to be administered systemically, is their short half-lives in the circulation [START_REF] Zasloff | Antimicrobial peptides in health and disease[END_REF]. Efforts are constantly being made to overcome this problem, with varying results [START_REF] Conlon | Potent and rapid bactericidal action of alyteserin-1c and its [E4K] analog against multidrug-resistant strains of Acinetobacter baumannii[END_REF].

Peptides have presented activity against A. baumannii [START_REF] Saugar | Activity of cecropin A-melittin hybrid peptides against colistin-resistant clinical strains of Acinetobacter baumannii: molecular basis for the differential mechanisms of action[END_REF]. Here we present three peptides that have recently been reported to exert activity against MDR-AB.

Alyteserin-1c and its [E4K] analogue

Alyteserin-1c is a 23-amino acid peptide that was first isolated from noradrenalinestimulated skin secretions from the midwife toad Alytes obstetricians [START_REF] Conlon | The alyteserins: two families of antimicrobial peptides from the skin secretions of the midwife toad Alytes obstetricans (Alytidae)[END_REF]. The haemolytic activity of the naturally occurring peptide was relatively low. Colnon et al. [START_REF] Conlon | Potent and rapid bactericidal action of alyteserin-1c and its [E4K] analog against multidrug-resistant strains of Acinetobacter baumannii[END_REF] reported that it presented bactericidal activity against MDR-AB in the MIC range of 5-10 M. Following substitution of the Glu4 residue by l-Lys, the created analogue [E4K] was found to have a four-fold increase in potency against all strains of MDR-AB tested by the authors [START_REF] Conlon | Potent and rapid bactericidal action of alyteserin-1c and its [E4K] analog against multidrug-resistant strains of Acinetobacter baumannii[END_REF]. Moreover, the haemolytic activity of the [E4K] analogue was appreciably lower than the naturally occurring peptide [lethal concentration for 50% of test population >400 M] and the analogue was extremely soluble in physiological buffers. In a time-kill assay, >99.9% of the bacteria were killed within 30 min by the [E4K] analogue at a concentration of 1 the minimum bactericidal concentration [START_REF] Conlon | Potent and rapid bactericidal action of alyteserin-1c and its [E4K] analog against multidrug-resistant strains of Acinetobacter baumannii[END_REF].

A c c e p t e d M a n u s c r i p t

Buforin II

Buforin II is a peptide isolated from the stomach of the Asian toad Bufo bufo gargarizans [START_REF] Park | Mechanism of action of the antimicrobial peptide buforin II: buforin II kills microorganisms by penetrating the cell membrane and inhibiting cellular functions[END_REF]. Cirioni et al. [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF] investigated the in vitro interactions and the in vivo efficacy of buforin II and RIF in a rat model of A. baumannii sepsis using both an A. baumannii ATCC 19606 strain and a MDR-AB clinical isolate [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF]. The two strains showed similar susceptibility to buforin II, with MICs of 8 mg/L, whereas synergy (fractional inhibitory concentration index of 0.312) was observed by testing buforin II combined with RIF. In the rat model of sepsis, following intraperitoneal injection of MDR-AB, the lethality rate in the control group was found to be 100% within 48 h compared with 46.6% for the group treated with buforin II, whereas a rate of 20.0% was observed in the combined group. Buforin II was also significantly superior to the control at reducing blood, spleen, peritoneum, liver and mesenteric lymph node complex bacterial burdens (P < 0.05). Furthermore, buforin II alone or combined produced a reduction of 46%, 20% and 28% in tumour necrosis factor (TNF), interleukin-6 and endotoxin plasma levels, respectively (P < 0.05). Overall, the combination of buforin II+RIF exhibited the highest antimicrobial activities and the strongest reduction in plasma endotoxin and cytokine levels. Similar results were also obtained when the experiments were performed using A. baumannii ATCC 19606 instead of the MDR-AB isolate. In conclusion, buforin II exerted strong antimicrobial activity, good survival rates and, finally, achieved a significant reduction in plasma endotoxin and cytokine concentrations. Moreover, synergism of the buforin II+RIF combination was observed. It appears that buforin II increases the permeability of the membrane thereby facilitating penetration of the otherwise impermeable hydrophobic solutes such as RIF [START_REF] Cirioni | Therapeutic efficacy of buforin II and rifampin in a rat model of Acinetobacter baumannii sepsis[END_REF]. The defensins are a group of -sheet-rich, cationic and amphipathic peptides that exert an antimicrobial effect, which appears to be achieved by creating pores or otherwise disrupting the cell membrane of target organisms, leading to the release of their cellular contents and ultimately destruction of the cells by osmolysis [START_REF] Pazgier | Human -defensins[END_REF]. hBD2 is primarily expressed in the epithelial lining of the human urinary and respiratory tracts [START_REF] Pazgier | Human -defensins[END_REF]. Routsias et al. [141] examined the bactericidal activity of hBD2 against epidemiologically unrelated hospital pathogens, of which 21 were A. baumannii isolates (14/21 isolates were MDR-AB). Preliminary data showed that incubation with 10 g/mL hBD2 for 2 h killed the entire A. baumannii population [141]. Notably, the 14 Acinetobacter isolates that exhibited a MDR phenotype were all (100%) sensitive to low concentrations of hBD2 with a virtually 99% lethal dose (vLD 99 ) of <10 g/mL, whilst the 7 non-MDR Acinetobacter isolates demonstrated an average vLD 99 of >10 g/mL. Furthermore, the vLD 90 ranged between 3.25 g/mL and 4.5 g/mL for all MDR strains, whilst the vLD 90 was significantly higher (ranging from 3.90 g/mL to 9.35 g/mL) for non-MDR strains (t = -3.74, P < 0.01) [141]. Other than mutations conferring resistance to certain antibiotics, it is possible that MDR strains also possess mutations that affect energy generation required for the maintenance of membrane integrity or affect the membrane structure itself, leading to supersusceptibility to the action of substances such as hBD2 [141].

Apart from its ability to fight MDR A. baumannii, hBD2 is an endogenous peptide of the innate immune system and may be less toxic than other molecules such as polymyxins. In addition to their direct antibacterial activity, defensins have the A c c e p t e d M a n u s c r i p t [START_REF] Rahal | Novel antibiotic combinations against infections with almost completely resistant Pseudomonas aeruginosa and Acinetobacter species[END_REF] potential to neutralise bacterial endotoxin by direct binding and inactivation of lipopolysaccharide, thereby preventing the ability of endotoxin to induce shock.

Furthermore, owing to their ability to form channels in lipid membranes, defensins facilitate the penetration of hydrophilic drugs.

A significant disadvantage of hBD2 is that the salt concentration affects its bactericidal effect. It was demonstrated that even 150 mM of sodium chloride was sufficient to decrease the bactericidal effect of hBD2 at least 15-fold, indicating that the maximum activity of hBD2 can only be found in sites of the human body with low salt concentration, such as the airway surface fluid of normal lung [141].

Conclusions

Currently, COL constitutes a salvage solution against MDR-AB. It is not as nephrotoxic as initially believed and should be used in combination with other antimicrobials, preferably MEM or RIF. Use of TIG against A. baumannii is off-label and its clinical efficacy remains a matter of controversy. Resistance to TIG during therapy may occur and TIG monotherapy should be avoided when the risk of pseudomonal infection is serious. TIG treatment should also be avoided in cases of bacteraemia by isolates with MICs of ≥1 g/mL. In cases of sulbactam-susceptible A. baumannii infections, sulbactam should be considered as the preferred therapeutic solution. RIF appears to exert bactericidal activity against MDR-AB. Its combination with other antimicrobials is crucial and the most active combination is considered to be that with COL. Minocycline may play a serious role in confronting minocyclinesusceptible MDR-AB infections, however clinical studies on this issue are rare.

A c c e p t e d M a n u s c r i p t [START_REF] Yoon | In vitro double and triple synergistic activities of polymyxin B, imipenem, and rifampin against multidrugresistant Acinetobacter baumannii[END_REF] Among the new agents, peptides appear to be the most promising. A series of recently reported peptides have shown strong bactericidal activity against MDR-AB.

However, various obstacles should be overcome before they can be used clinically.

  al. (MIC 90 at 2 mg/L) [65], Souli et al. (MIC 90 of MDR-AB at 1 mg/L) [38] and Hoban et al. (MIC 90 of MDR-AB at 2 mg/L) [66].

3 .

 3 Human -defensin2 (hBD2)

  s c r i p t

Funding

None.

Competing interests

None declared.

Ethical approval

Not required.

A c c e p t e d M a n u s c r i p t