Rafael Gomes

Muriel Primon Borowski

Denise Barros

Norberto Peporine Brentan Da Silva

Karine Rigon Lopes

Charley Christian Zimmer

Cristiane Staats

Emmanuel Bernardes De Oliveira

Reynald Giudice

Alexandre José Gillet

Macedo

Borowski

Muriel Primon Barros

Denise Brentan Da Silva

Norberto Peporine Lopes

Karine Rigon Zimmer

Charley Christian Staats

Cristiane Bernardes De Oliveira

Emmanuel Giudice

Reynald Gillet

Alexandre José Macedo

Simone Cristina

Baggio Gnoatto

Aline Rigon Zimmer
email: aline.zimmer@ufrgs.br

Red pepper peptide coatings control Staphylococcus epidermidis adhesion and biofilm formation

Keywords: biofilm, antibiofilm, antiadhesive, capsicum, peptide, coating

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Medical devices (indwelling) have made great contributions to health and quality of life evolution. [START_REF] Lim | Development of a catheter functionalized by a polydopamine peptide coating with antimicrobial and antibiofilm properties[END_REF][START_REF] Shah | Current status of the regulation for medical devices[END_REF] However, infections related to the use of these medical devices and to widespread antibiotic misuse have led to a lethal health crisis with the increasing appearance of tolerant and resistant bacteria that do not respond to conventional antibiotics. [START_REF] Levin-Reisman | Antibiotic tolerance facilitates the evolution of resistance[END_REF][4][START_REF] Menegueti | Long-term prevention of catheter-associated urinary tract infections among critically ill patients through the implementation of an educational program and a daily checklist for maintenance of indwelling urinary catheters: A quasiexperimental study[END_REF] Associated with this phenomenon is the ability of microorganisms to form biofilms that confer evolutionary advantages that are phenotypically beneficial to increased survival and symbiotic relationships. [START_REF] Olsen | Biofilm-specific antibiotic tolerance and resistance[END_REF] A biofilm is a complex matrix composed of assembled extracellular polymeric substances (EPSs) enwrapping microorganisms. This structure allows bacterial surface fixation on catheters, implants, probes, prostheses or damaged tissues and protects bacteria against antibiotics and host immune defenses. [START_REF] Davies | Understanding biofilm resistance to antibacterial agents[END_REF][START_REF] Romling | Biofilm infections, their resilience to therapy and innovative treatment strategies[END_REF][START_REF] Haussler | Biofilms 2012: New Discoveries and Significant Wrinkles in a Dynamic Field[END_REF][START_REF] Hall | Molecular mechanisms of biofilm-based antibiotic resistance and tolerance in pathogenic bacteria[END_REF][START_REF] Post | Comparative Genomics Study of Staphylococcus epidermidis Isolates from Orthopedic-Device-Related Infections Correlated with Patient Outcome[END_REF] Biofilm formation is considered a significant virulence factor and is present in 80% of difficult-to-treat human infections, such as endocarditis, osteomyelitis, urethritis, and periodontitis, strongly impacting patient outcomes and health system costs. 12- 16 In this context, Staphylococcus epidermidis is an opportunistic pathogen that is highly associated with medical device-related infections. [START_REF] Chusri | Inhibition of Staphylococcus epidermidis Biofilm Formation by Traditional Thai Herbal Recipes Used for Wound Treatment. Evidence-Based Complementary and Alternative Medicine[END_REF][START_REF] Laverty | Biomolecular mechanisms of staphylococcal biofilm formation[END_REF][START_REF] Chessa | Staphylococcus aureus and Staphylococcus epidermidis Virulence Strains as Causative Agents of Persistent Infections in Breast Implants[END_REF][START_REF] Vassallo | Shortcourse daptomycin lock and systemic therapy for catheterrelated bloodstream infections: a retrospective cohort study in cancer patients with surgically implanted devices[END_REF][START_REF] Oliveira | Staphylococcus aureus and Staphylococcus epidermidis infections on implants[END_REF][START_REF] Lee | Global spread of three multidrug-resistant lineages of Staphylococcus epidermidis[END_REF] Current discussions suggest that this bacterium should be classified as pathogenic because disease-causing S. epidermidis strains represent a pathogenic subpopulation that has acquired genetic elements and related phenotypes that promote infection. [START_REF] Méric | Diseaseassociated genotypes of the commensal skin bacterium Staphylococcus epidermidis[END_REF] Furthermore, the ability to form biofilms on diverse devices or surfaces is the major virulence factor of S. epidermidis. [START_REF] Otto | Staphylococcal biofilms[END_REF] For instance, biofilm-forming S. epidermidis strains have been reported to include many strains that are resistant to ciprofloxacin and sulfamethoxazole, as well as to aminoglycosides. [START_REF] Li | Bacteria antibiotic resistance: New challenges and opportunities for implant-associated orthopedic infections[END_REF] Some technological strategies for the development of biomaterials that have been proposed include functionalized or coated antiadhesive surfaces and intrinsically antibacterial or nanostructured materials. [START_REF] Campoccia | A review of the biomaterials technologies for infection-resistant surfaces[END_REF][START_REF] Ashton | In vitro testing of a first-in-class trialkylnorspermidine-biaryl antibiotic in an anti-biofilm silicone coating[END_REF][START_REF] Gao | Rationally designed dual functional block copolymers for bottlebrush-like coatings: In vitro and in vivo antimicrobial, antibiofilm, and antifouling properties[END_REF] However, among these strategies, the use of natural products to control bacterial adhesion and biofilm formation is rare. [START_REF] Trentin | Natural Green Coating Inhibits Adhesion of Clinically Important Bacteria[END_REF] Peppers from the genus Capsicum comprise more than 200 varieties, and their fruits are broadly diverse in relation to size, shape, flavor and sensory heat. [START_REF] Qin | Whole-genome sequencing of cultivated and wild peppers provides insights into Capsicum domestication and specialization[END_REF] Capsicum species are native to the tropical and humid zones of Central and South America and include peppers of important economic value. [31][START_REF] Menichini | The influence of fruit ripening on the phytochemical content and biological activity of Capsicum chinense Jacq. cv Habanero[END_REF][START_REF] Kim | Genome sequence of the hot pepper provides insights into the evolution of pungency in Capsicum species[END_REF] They are commonly consumed as spice or food as well as used as remedies in Indian, Native American and Chinese traditional medicine to treat skin rashes, dog/snake bites and wounds. [START_REF] Meghvansi | Naga chilli: A potential source of capsaicinoids with broad-spectrum ethnopharmacological applications[END_REF] Capsicum peppers display several biological activities, such as anti-inflammatory, hypoglycemic and hypocholesterolemic, though its pharmacological potential and chemical composition have been underexplored. [START_REF] Srinivasan | Lycopene: An antioxidant and radioprotector against gammaradiation-induced cellular damages in cultured human lymphocytes[END_REF][START_REF] Zimmer | Antioxidant and anti-inflammatory properties of Capsicum baccatum: From traditional use to scientific approach[END_REF][START_REF] Zimmer | Long-Term Oral Administration of Capsicum baccatum Extracts Does Not Alter Behavioral, Hematological, and Metabolic Parameters in CF1 Mice[END_REF][START_REF] Ahuja | The effect of 4-week chilli supplementation on metabolic and arterial function in humans[END_REF][START_REF] Aizawa | Dietary capsanthin, the main carotenoid in paprika (Capsicum annuum), alters plasma highdensity lipoprotein-cholesterol levels and hepatic gene expression in rats[END_REF][START_REF] Galvez Ranilla | Phenolic compounds, antioxidant activity and in vitro inhibitory potential against key enzymes relevant for hyperglycemia and hypertension of commonly used medicinal plants, herbs and spices in Latin America[END_REF][START_REF] Liu | Capsaicinoids in the Hottest Pepper Bhut Jolokia and its Antioxidant and Antiinflammatory Activities[END_REF][START_REF] Sharma | Mechanisms and clinical uses of capsaicin[END_REF] Recently, we demonstrated that an aqueous extract of C. baccatum seeds decreased S. epidermidis biofilm formation by 80%, indicating that this red pepper is a potential source of antibiofilm and antiadhesive compounds. [START_REF] Gomes | Red pepper Capsicum baccatum : source of antiadhesive and antibiofilm compounds against nosocomial bacteria[END_REF] In the present study, we purified and identified antibiofilm and antiadhesive peptides from C. baccatum using chromatography-tandem mass spectrometry, MALDI-MS/MS and bioinformatics tools. We demonstrated the ability of Capsicum storage peptides (CSP) to prevent biofilm formation by methicillin-resistant S. epidermidis without antibiotic mechanisms. CSP cytotoxic profiles were assessed in vitro. Surface CSP coatings were developed, and their effects were examined and characterized. These results may corroborate the development of new natural strategies in the war against medical device-related infections.

Material and Methods

Chemicals

All reagents, which were analytical or HPLC grade, were purchased from Sigma Chemical Company™. All solvents were purchased from Vetec™ AG (Rio de Janeiro, Brazil). The matrices 2,5-dihydroxybenzoic acid (DHB) and sinapinic acid (SA) were purchased from Bruker Daltonics. Synthetic peptides suitable for use in cell culture were purchased from China Peptides with >95% purity.

Plant material and extraction

C. baccatum var. pendulum (Willd.) Eshbaugh (Solanaceae) fruits were obtained from a cultivated area in Turuçu, Rio Grande do Sul (RS), Brazil (Figure S1, Supplementary Material). A voucher specimen (ICN 181469) was identified and deposited at the Herbarium of Universidade Federal of Rio Grande (UFRGS, Brazil). The seeds and extracts were obtained and prepared according to Gomes Von Borowski et al. [START_REF] Gomes | Red pepper Capsicum baccatum : source of antiadhesive and antibiofilm compounds against nosocomial bacteria[END_REF] The most active extract was lyophilized to obtain the residual aqueous extract from seeds (RAqS).

Purification of residual aqueous extract from seeds (RAqS)

RAqS (700 mg) were solubilized in ultrapure water and subjected to column chromatography (3 cm diameter) using Sephadex™ LH-20 (40 g). Water was used as the eluent, and fourteen fractions of 10 ml each were collected (F1-F14). These purified fractions were screened for antibiofilm and antibacterial activities against S. epidermidis at 4.0, 2.0, 1.0, 0.5 and 0.25 mg/mL. The selected active fraction, which presented 1.4% yield, was submitted to characterization by LC-DAD-ESI-MS and MALDI-MS/MS.

Bacterial strain and culture conditions

Staphylococcus epidermidis (ATCC 35984) was cultivated on Muller Hinton agar at 37ºC overnight. The inoculum concentration was adjusted to an optical density at 600 nm equivalent to 10 8 CFU/ml.

Sample preparation for microbiological assays

Samples were freshly dissolved in ultrapure water and filtered through a 0.2 µm pore size membrane before each assay. The red pepper active fraction was tested at 2.0 mg/ml and the synthetic peptide (CSP37) at 0.2, 0.4 and 0.9 mM.

Antibiofilm and antibacterial assays

To evaluate biofilm formation, an adapted protocol from Trentin et al. [START_REF] Trentin | Natural Green Coating Inhibits Adhesion of Clinically Important Bacteria[END_REF] was applied using 0.4% crystal violet staining and standard sterile 96-well polystyrene flat-bottom microtiter plates. Finally, the absorbance (570 nm) was measured (Spectramax™ M2e Multimode Microplate Reader, Molecular Devices, Sunnyvale, CA, USA). Bacterial growth was monitored by absorbance at 600 nm. Biofilm formation in the presence of the water control was considered to represent 100% biofilm formation.

Kinetics of bacterial growth

Bacterial growth kinetics were evaluated by monitoring the optical density (600 nm) at different timepoints. Samples were monitored after 0, 1, 3, 6, 12, and 24 h of incubation (37°C). The red pepper active fraction (2.0 mg/ml) was evaluated, and rifampicin (8 µg/ml) was used as a death control. The results were expressed as OD600nm per incubation time.

Coatings preparation and characterization

Solutions of the red pepper active fraction were prepared using 70% methanol and filtered through a 0.2 µm pore size membrane. Permanox™ slides (30 x 25 mm) were coated once or three times with the active fraction solution at either 2.0 mg/ml or 10.0 mg/ml. Methanol was used to prepare nontreated control slides. In a Laurell model WS-650MZ-23NPP/LITE spin coater, 300 µl of the active fraction solution was deposited on each Permanox™ slide. It was accelerated for 5 s up to an angular velocity of 500 rpm and subsequently accelerated to 4000 rpm for 40 s. To remove the remaining solvent and to promote film annealing, slides were heat-treated at 40ºC in a stove for 4 h. After the coating process, slides were sterilized using UV light for 20 min, cut to 10 x 25 mm and used in antibiofilm assays and scanning electron microscopy evaluation. Permanox™ samples were characterized before and after coating by water contact angle (WCA), which was measured using the sessile drop technique and Milli-Q water.

Drop images were captured and analyzed by a Theta Lite optical tensiometer (Attension). The reported WCAs are the means of five measurements performed on different areas of each sample surface.

Scanning electron microscopy (SEM)

Biofilms of S. epidermidis were cultured as described above. One group received a piece of noncoated Permanox™ slide (as a nontreated and no solvent control), while others received a piece of red pepper active fraction-coated slide (once or three times at 2.0 mg/ml or 10.0 mg/ml) or methanol-coated slide (as a solvent control). Noncoated controls were incubated with TSB broth containing the active fraction and bacteria, and coated samples were incubated with only TSB broth and bacteria (24 h at 37°C). Then, the slides were washed with sterile 0.9% NaCl and fixed with 2.5% glutaraldehyde (prepared in 100 mM cacodylate buffer, pH 7.2). Afterwards, they were washed with 100 mM cacodylate buffer (pH 7.2), dehydrated with increasing concentrations of acetone, dried with CO2 as the critical point (Bal-Tec CPD 030 critical point dryer, Balzers, Liechtenstein) and placed on a mounting base. Finally, they were coated with carbon and examined under a scanning electron microscope (model JSM-6060, JEOL) at 10 kV.

Hemolysis test

Hemolysis assays were performed with human venous blood from healthy donors as previously described by Rocha et al. [START_REF] Rocha | Anti-Trichomonas vaginalis activity of saponins from Quillaja, Passiflora, and Ilex species[END_REF] All consents, procedures and documents were approved by the Ethics Committee of Universidade Federal do Rio Grande do Sul (UFRGS) (number 666.655/2014). The red pepper active fraction was tested at 2.0 mg/ml. A solution of 1% Triton™ was used to produce 100% hemolysis (reference, positive control), and phosphate buffered saline (PBS) was used as a negative control (no hemolysis). The test sample results were statistically compared with those of the positive and negative controls.

Lymphocyte cytotoxicity test and LDH release determination

Lymphocytes were obtained from heparin-treated human venous blood from healthy donors and isolated as previously described by Waetchter et al. [START_REF] Waechter | Design, Synthesis and Biological Evaluation of Betulinic Acid Derivatives as New Antitumor Agents for Leukemia[END_REF] Briefly, the lymphocytes were separated by Histopaque™ gradient centrifugation, cultured at 100,000 cells/well in a 24-well plate in RPMI 1640 medium containing 10% fetal bovine serum (FBS) (Gibco BRL, Grand Island, NY), and treated with either the red pepper active fraction at 2 mg/ml, 1% Triton™ solution (cytotoxic control) or RPMI medium (nontoxic control) for 24 h, all in quintuplicate. Cell morphology was verified by optical microscopy, and viable cell counting was performed with a FACSVerse™ flow cytometer equipped with a 488 nm blue laser and a flow sensor (BD Biosciences, San Jose, CA, EUA). BD FacSuite™ software (BD Biosciences) was used for data analysis. All consents, procedures and documents were approved by the Ethics Committee of UFRGS (number 666.655/2014). To monitor lactate dehydrogenase (LDH) release due to lysis, the supernatants of the treated cells were assessed for LDH activity using the LDH enzymatic assay kit available from Laborclin™ (São José do Rio Preto, São Paulo, Brazil). The results are expressed as percentages, and the Triton treatment was considered to result in 100% LDH release.

High-throughput multiparameter cell image analysis

High Content Screening (HCS) and High Content Analysis (HCA) were performed with a robotic platform (ImPACcell, BIOSIT, Université de Rennes 1) using 7 representative human cell lines: HuH7 (liver), CaCo-2 (colon), MDA (breast), HCT116 (colon), PC3 (prostate), NCI-H727 (lung) and MCF7 (skin). The number of normal cells is presented as the residual cell percentage (%) compared to the DMSO control average, and the synthetic peptide (CSP37) was tested at 25 µM. [START_REF] Richy | Structure-based design of human immuno-and constitutive proteasomes inhibitors[END_REF][START_REF] Castillo | Synthesis and toxicity evaluation of hydrophobic ionic liquids for volatile organic compounds biodegradation in a two-phase partitioning bioreactor[END_REF] The platform is equipped with an Olympus upright microscope (Spot NB camera and Simple PCI software, Compix), a Zeiss AxioImager M1 upright microscope (Marzhauser, Zeiss NB camera and AxioVision software), an ArrayScan VTI high-content imager (Thermo/Cellomics), Hamilton STARlet and Nimbus liquid handling systems, and a Scienion liquid spotter.

Liquid chromatography-tandem mass spectrometry analysis

Analyses were performed on a Shimadzu UFLC-20AD system coupled to a diode array detector (DAD) and an ESI-ion trap mass spectrometer (LC-DAD-ESI-MS) (AmaZon SL, Bruker Daltonics). A Luna C18 column (5 µm, 4,6 mm x 250 mm, Phenomenex) was used for chromatography analyses. The mobile phase consisted of acetonitrile (solvent B) and water (solvent A), each containing 0.1% formic acid. The flow rate was 0.6 ml/min, and the injection volume was 20 µl. The elution profile was the following: 2% B (0 to 3 min), 2 to 20% B (3 to 22 min), 20 to 30% B (22 to 30 min), 30 to 100% B (30 to 40 min), and 100% B (40 to 45 min). The LC flow was split 3:7. The MS conditions were the following: the capillary voltage was 3500 V, nebulizer gas (nitrogen) pressure was set to 50 psi, dry gas flow was 9 l/min, and dry temperature was 300°C. MS data were acquired in negative and positive ion modes. The samples were analyzed over a mass range (m/z) of 0-1200.

MALDI-MS/MS analyses

UltrafleXtreme MALDI-TOF/TOF equipment (BrukerDaltonics, Bremen, Germany) was used for the analyses. For analyses of the fractions, the matrices 2,5-dihydroxibenzoic acid (DHB) and sinapinic acid (SA) were evaluated, and better results were obtained with DHB, which was used for all protein analyses. MS analysis: the samples were analyzed by reflector (700 to 5000 Da and 4 to 20 kDa) and linear modes (4-20, 20-50 and 50-100 kDa). The samples were dissolved in acetonitrile (ACN):H2O (20:80, v/v) with 0.1% trifluoroacetic acid and the DHB matrix (20 mg/ml) in ACN:H2O (30:70, v/v) with 0.1% trifluoroacetic acid. The sample solution and the matrix were mixed in equal amounts and spotted onto a ground stainless steel MALDI target (1 µl). The experimental conditions for MS analyses were as follows: pulsed ion extraction of 360 ns, laser frequency of 1000 Hz, and positive ion mode. In all, 5000 shots were averaged to record each mass spectrum. For the MS/MS analyses, the red pepper active fraction was resolved by electrophoresis gel and the obtained spots (Sp1, Sp2 and Sp3) were digested with trypsin. For these analyses, a saturated solution of α-cyano-4-hydroxycinnamic acid (CCA) prepared in ACN:H2O (30:70, v/v) with 0.1% trifluoroacetic acid was used. The samples were dissolved in ACN:H2O (30:70, v/v) with 0.1% trifluoroacetic acid. The sample solution and the matrix were mixed in equal amounts and spotted onto a ground stainless steel MALDI target (1 µl). The peptide fractions were analyzed by MS and MS/MS. For MS analyses, the experimental conditions used were as follows: pulsed ion extraction of 100 ns, laser frequency of 1000 Hz, reflectron mode, and positive ion mode. In all, 600 shots were averaged to record each mass spectrum. The selected ions were accelerated to 19 kV on the LIFT cell for MS/MS analyses. External calibrations were performed using peptide calibration standards (angiotensin II and I, substance P, bombesin, ACTH clip 1-17, ACTH clip 18-39 and somatostatin) and protein standards I (insulin, ubiquitin I, cytochrome C and myoglobin) and II (trypsinogen, protein A and bovine serum albumin (BSA)) from Bruker Daltonics.

Database identification and CSP37 prediction

Database identification: MS/MS spectra were analyzed and converted to mfg files using Mascot Distiller (MATRIX SCIENCES, London, UK). Protein identification was performed by database searches with a local Mascot Server v 2.3 (MATRIX SCIENCES, London, UK) against the proteome sequence of Capsicum anuum. [START_REF] Kim | Genome sequence of the hot pepper provides insights into the evolution of pungency in Capsicum species[END_REF] The parameters used for identification allowed a maximum of one missed tyrosine cleavage, the carbamidomethylation of cysteine, and the possible oxidation of methionine. The peptide MS/MS tolerance was 0.25 Da, and the significance threshold was set at p < 0.05. The sequences identified by MASCOT as significant hits were annotated using the NCBI BLAST program and the NCBI Conserved Domain Database. CSP37 prediction: since the genome/proteome of C. baccatum is not fully sequenced, we used an alternative strategy to identify the principal peptides predicted by Mascot, which led to the identification of CSP37. Proteins containing the predicted peptides were identified by using P-BLAST to search a database of annotated Capsicum sequences. This led to the identification of two proteins (XP_016559134.1, 2S albumin-like [Capsicum annuum] and XP_016567465.1, 2S sulfur-rich seed storage protein 2-like [Capsicum annuum]) with very strong sequence identity. We then realigned the two proteins and the Mascot peptides using Clustal Omega. We discovered that the two peptides were in fact two consecutive segments (separated by a single residue) of a single longer peptide (CSP37), whose mass indeed corresponded to the most intense peak of the MS spectrum.

Statistical analysis

The results are expressed as the mean ± standard deviation. Biofilm and growth data were analyzed using Student's t-test, and p-values <0.05 were considered statistically significant. Cytotoxicity data were analyzed using ANOVA followed by Tukey's post hoc test, and p-values <0.05 were considered statistically significant.

Results

Bioguided fractionation and identification of antibiofilm peptides from C. baccatum

Previously, we obtained a protein extract from C. baccatum seeds (RAqS) with potential antibiofilm activity. [START_REF] Gomes | Red pepper Capsicum baccatum : source of antiadhesive and antibiofilm compounds against nosocomial bacteria[END_REF] Herein, further purification steps were performed in order to obtain the bioactive fraction from RAqS responsible for antibiofilm activity. After chromatographic collection of 14 fractions, the ninth fraction was determined to be the one containing the biological activity (Figure S2). Once identified, this antibiofilm active fraction was subjected to chemical elucidation. Briefly, MALDI-MS analyses did not show ions up to m/z 4000, but some ions were observed in the range of 4 to 20 kDa (Figure 1), characteristic of peptides abundant in Capsicum. [START_REF] Lee | Involvement of the pepper antimicrobial protein CaAMP1 gene in broad spectrum disease resistance[END_REF][START_REF] Dias | Isolation, Characterization and Antifungal Activity of Proteinase Inhibitors from Capsicum chinense Jacq. Seeds[END_REF] The m/z 4305 ion showed the highest relative ion intensity in the mass spectrum of the red pepper active fraction, and less intense ions were observed at m/z 4220, 4364, 9372 and 9415. No ions were observed in the mass range 20-100 kDa, moreover, LC-DAD-ESI-MS analysis did not reveal secondary metabolites of low molecular weight. Thus, the red pepper active fraction was subjected to gel electrophoresis, and the obtained spots (Sp1, Sp2 and Sp3) were analyzed by MALDI-MS and MS/MS to identify the protein precursors. All spots were found to contain peptides belonging to the same protein family, the 2S sulfur-rich seed storage protein (2SS protein) family, according to the NCBI CDD and BLAST searches using the proteome of C. annum as the database (the C. baccatum proteome is unknown) (Figure 1, Table 1). Therefore, the red pepper active fraction is composed of peptides derived from a 2SS protein (C. baccatum) and will be referred to herein as "Capsicum storage peptides (CSP)".

CSP control methicillin-resistant S. epidermidis biofilm formation

Staphylococcus epidermidis biofilm formation was completely prevented in the presence of CSP solution (2.0 mg/mL) (Figure 2A). Remarkably, CSP did not influence bacterial growth at the same concentration (Figure 2A). In addition, bacterial growth kinetics confirmed the absence of any antibiotic effect of CSP (Figure 2B). Although biofilm formation was prevented, the planktonic cells remained viable, and the colonies were phenotypically normal.

Figure 2

Red pepper peptide coatings strongly prevent S. epidermidis adhesion

To verify the antiadhesive capability of CSP, we applied the spincoat technique to coat Permanox™ slides with CSP (herein called "red pepper coat"), exposed the slides to S. epidermidis cultures and analyzed them by scanning electron microscopy (SEM) after 24 h of exposure (Figure 3A-J). Red pepper-coated surfaces prevented bacterial adhesion, aggregation and accumulation (Figure 3C-J). In addition, the noncoated control that received 2.0 mg/mL soluble CSP in the culture medium did not display any adherent bacteria (Figure S8, Supplementary Material). Compared to controls, bacterial adhesion and aggregation profiles decreased on slides that had been coated once with 2.0 mg/ml CSP (Figure 3C-D). Slides that had been coated three times with 2.0 mg/ml CSP show only a few individual cells, although extracellular matrix was not detected (Figure 3G-H). Likewise, slides coated with a higher CSP concentration (10.0 mg/ml) displayed a complete absence of bacteria and extracellular matrix (Figure 3E-F, I-J). Control slides (vehicle-coated and noncoated slides without CSP solution) were not able to prevent the bacterial adhesion, aggregation and accumulation that facilitate biofilm formation (Figures 3A-B andS3, Supplementary Material).

Red pepper peptide coatings create a hydrophilic surface on coated material

To evaluate the hydrophilicity and the quality of red pepper coatings produced by spin coating (Figure 3k), we used the water contact angle (WCA) image and measurement analysis. Red pepper coatings displayed hydrophilic characteristics in a concentration-dependent manner compared to controls. The surfaces singly and triply coated with 2.0 mg/mL CSP displayed WCAs of 75º and 81º, respectively. At 10.0 mg/mL, the WCA decreased to 26º and 18º for singly and triply coated surfaces, respectively. However, noncoated and methanol-coated surfaces exhibited hydrophobic characteristics, displaying WCAs of 87º and 92º, respectively.

CSP37 is the lead antibiofilm peptide from the CSP fraction

We postulated that the peptide detected with higher intensity in the CSP active fraction (m/z 4289, Figure 1) would mediate CSP bioactivity. Thus, we performed sequence and structural predictions of this major peptide of CSP. For this purpose, we used the CSP MS/MS data and sequence alignment analysis to determine its sequence: RSCQQQIQQAQQLSSCQQYLKQRVQSEEGEDQISQRE (European Patent n°EP19305205) (Figure 4A-C). The predicted peptide was named "Capsicum storage peptide 37 (CSP37)" and was subsequently synthesized and experimentally tested. CSP37 prevented S. epidermidis biofilm formation independently of growth inhibition, equivalently to CSP (Figure 4D). Their bioactivities (Antibiofilm, non-antibiotic and non-cytotoxic) support the correlation between them (Figure 4D, Figure 2A and Figure 5).

Red pepper peptides are not cytotoxic

To evaluate CSP cytotoxicity, human erythrocytes and lymphocytes were used as model cells. CSP at 2.0 mg/ml displayed a slight hemolytic activity (11% hemolysis), preserving the erythrocytes integrity similarly to the PBS control (Figure 5B). Equally, CSP did not present cytotoxic effects against lymphocytes after 24 h of exposure (7.7% decrease in cell counts) (Figures 5A, C-E). Furthermore, the LDH assay indicated that CSP did not affect lymphocyte membrane integrity (LDH release by Triton treatment was considered 100% for quantification, the normal control showed 17.3%, and CSP 14.7%). Likewise, high content analysis (HCA) showed no CSP37 cytotoxicity against 7 different representative human cell lines (Figure 5F).

Discussion

Biofilm-forming S. epidermidis is an important pathogen worldwide and is closely linked with medical device-and immunosuppression-associated infections. [START_REF] Lourtet-Hascoët | Staphylococcus lugdunensis, a serious pathogen in periprosthetic joint infections: comparison to Staphylococcus aureus and Staphylococcus epidermidis[END_REF][START_REF] Widerström | Significance of Staphylococcus epidermidis in Health Care-Associated Infections, from Contaminant to Clinically Relevant Pathogen: This Is a Wake-Up Call![END_REF][START_REF] Gabriel | Characterisation of clinical meticillin-resistant Staphylococcus epidermidis demonstrating high levels of linezolid resistance (>256 μg/ml) resulting from transmissible and mutational mechanisms[END_REF] The present study demonstrates that natural Capsicum storage peptides (CSP) exhibit potent antibiofilm activity against methicillinresistant S. epidermidis by a mechanism that does not involve cell killing. The study presents the identification of CSP37, the major peptide of CSP and a source of its antibiofilm activity. Strikingly, the results of our coating experiments support the antiadhesive applicability of these peptides. Plants are constantly exposed to a large variety of pathogenic microorganisms, and they produce natural compounds as resistance factors, such as constitutive peptides and proteins. 54- 56 Peptides are a very common chemical component of Capsicum seeds. [START_REF] Dias | Isolation, Characterization and Antifungal Activity of Proteinase Inhibitors from Capsicum chinense Jacq. Seeds[END_REF][START_REF] Lee | Molecular characterization of a cDNA for a cysteine-rich antifungal protein from Capsicum annuum[END_REF] In this context, some studies have suggested that plant-derived peptide fractions exhibit antiadhesive properties. [START_REF] Gomes | Red pepper Capsicum baccatum : source of antiadhesive and antibiofilm compounds against nosocomial bacteria[END_REF][START_REF] Lengsfeld | Glycosylated compounds from okra inhibit adhesion of Helicobacter pylori to human gastric mucosa[END_REF][START_REF] Wittschier | Large molecules as anti-adhesive compounds against pathogens[END_REF][START_REF] Bensch | Investigations into the antiadhesive activity of herbal extracts against Campylobacter jejuni[END_REF] Recent literature reviews have shown that peptides and peptidomimetics from different sources have been investigated for their prominent antibiofilm activity. [START_REF] Borowski | Peptides as a strategy against biofilm-forming microorganisms: Structureactivity relationship perspectives[END_REF][START_REF] Gomes | Promising Antibiofilm Activity of Peptidomimetics[END_REF][START_REF] Batoni | Antimicrobial peptides and their interaction with biofilms of medically relevant bacteria[END_REF][START_REF] De La Fuente-Nunez | D-Enantiomeric Peptides that Eradicate Wild-Type and Multidrug-Resistant Biofilms and Protect against Lethal Pseudomonas aeruginosa Infections[END_REF][START_REF] Pfalzgraff | Antimicrobial Peptides and Their Therapeutic Potential for Bacterial Skin Infections and Wounds[END_REF] There are no previous studies demonstrating a relationship between 2S sulfur-rich seed storage (2SS) proteins or peptides and antibiofilm activity. To the best of our knowledge, this is the first scientific report using Capsicum natural peptides as antibiofilm and antiadhesive agents, herein called red pepper coat. The purification of C. baccatum extract allowed the chemical identification of the active fraction able to prevent S. epidermidis biofilm formation. This antibiofilm fraction is composed of peptides from a Capsicum 2SS protein, herein named Capsicum storage peptides (CSP). Hence, the reconstitution of the major MS peak of the CSP spectrum allowed the identification of its main peptide (CSP37, European Patent n° EP19305205), consisting of a 37 amino acid sequence that is also present in the 2SS protein described in the Capsicum genus. The NCBI database describes the 2SS protein and the 2S albumin-like protein as members of the Alpha-Amylase Inhibitors (AAIs) and Seed Storage (SS) Protein subfamilies. They are present in the seeds of a variety of plants and are widely distributed in dicot seeds, and they play an important role in plant natural defenses (NCBI accession: XP_016567465.1, CAA01774.1, NP_001295683.1, NP_001078429.1). Despite differences in their subunit structure and synthesis, all are compact globular proteins with conserved cysteine residues. [START_REF] Shewry | SEED STORAGE PROTEINS -STRUCTURES AND BIOSYNTHESIS[END_REF] Notably, both CSPs and CSP37 strongly decrease S. epidermidis biofilm formation without antibiotic effects. These results establish that the antibiofilm activity of CSP and CSP37 utilizes a mechanism of action that is independent of cell killing and of antibiotic mechanisms. Accordingly, nonantibiotic strategies have emerged as alternatives to combat bacterial resistance and tolerance, such as the targeting of extracellular bacterial virulence factors. This strategy is very important because it is less likely to impact bacteria gene machinery, and it may include antibiofilm agents to reduce bacteria adhesion. [START_REF] Travier | Escherichia coli Resistance to Nonbiocidal Antibiofilm Polysaccharides Is Rare and Mediated by Multiple Mutations Leading to Surface Physicochemical Modifications[END_REF][START_REF] Dostert | Design and Assessment of Anti-Biofilm Peptides: Steps Toward Clinical Application[END_REF][START_REF] Sun | Nanotechnology-enabled materials for hemostatic and antiinfection treatments in orthopedic surgery[END_REF][START_REF] Brannon | The arsenal of pathogens and antivirulence therapeutic strategies for disarming them[END_REF][START_REF] Totsika | Disarming pathogens: benefits and challenges of antimicrobials that target bacterial virulence instead of growth and viability[END_REF][START_REF] Silva | Plant Natural Products Targeting Bacterial Virulence Factors[END_REF] In this sense, the prototype red pepper coat (CSP spin-coated slides) prevented bacterial adhesion, aggregation, accumulation and consequently biofilm establishment. This activity seems to be concentration dependent. The antiadhesive effect of red pepper coatings may be due to multiple causes, including electrostatic interactions and steric hindrance. Repulsion forces may act between the anionic Grampositive bacterial surface and biofilm matrix and the electronegative field of the coated surface (Figure 6). The film produced by CSP coating may also give rise to steric hindrance that blocks bacterium-substratum interactions. [START_REF] Trentin | Natural Green Coating Inhibits Adhesion of Clinically Important Bacteria[END_REF] Moreover, CSP are mainly composed of polar amino acids, leading to important hydrophilic characteristics. Supported by angle contact analysis, red pepper-coated surfaces presented increased hydrophilicity on the entire coated surface compared to controls (hydrophobic material), showing that CSP occupy and alter the hydrophobic characteristics of Permanox™. Bacterial attachment is initially dependent on their hydrophobic and electrostatic physicochemical characteristics as well as on the surfaces involved; thus, the hydrophilic character of red pepper coat contributes to a reduction in these interactions. [START_REF] Trentin | Natural Green Coating Inhibits Adhesion of Clinically Important Bacteria[END_REF][START_REF] Nostro | Study on carvacrol and cinnamaldehyde polymeric films: mechanical properties, release kinetics and antibacterial and antibiofilm activities[END_REF][START_REF] Otto | Staphylococcal Infections: Mechanisms of Biofilm Maturation and Detachment as Critical Determinants of Pathogenicity[END_REF] Figure 6 In addition, both activities remain, even after 24 h under the incubation conditions. The spin-coating technique anchors substances on surfaces and is appropriate for the study of these coatings during transient or short-term exposure, which is sufficient for establishing proof of concept that natural products may act against pathogenic bacterial adhesion, although stronger interactions such as covalent attachment are required to extend coating lifetime for long-term studies. Importantly, CSP was not cytotoxic to human erythrocytes and lymphocytes. CSP37, the proposed major peptide of CSP, showed no cytotoxicity against 7 representative human cell lines. Furthermore, previous studies with C. baccatum corroborate this favorable toxicological profile. For example, RAqS, the precursor of CSP, was not toxic to Galleria mellonella larvae [START_REF] Gomes | Red pepper Capsicum baccatum : source of antiadhesive and antibiofilm compounds against nosocomial bacteria[END_REF] which presents similarities to the innate immune response of mammals like humoral and cellular components, and other extracts from C. baccatum displayed a good level of pharmacological safety during 60 days of oral administration in mice. [START_REF] Zimmer | Long-Term Oral Administration of Capsicum baccatum Extracts Does Not Alter Behavioral, Hematological, and Metabolic Parameters in CF1 Mice[END_REF] These results suggest that red pepper coatings do not affect either eukaryotic or prokaryotic membranes. 74,[START_REF] Strempel | Potential Application of Antimicrobial Peptides in the Treatment of Bacterial Biofilm Infections[END_REF]

Conclusions

In conclusion, this study shows that red pepper peptide coatings are a promising antibiofilm and antiadhesive alternative to prevent methicillin-resistant S. epidermidis colonization. Importantly, components of red pepper coatings are not cytotoxic and act by a nonantibiotic mechanism of action. In addition, CSP37 may facilitate research for innovative molecular designs as a prime structure. Importantly, this study sheds light on the use of natural products and their derivatives, which could bring innovative solutions for materials science. The identification of novel peptides aims to overcome the lack of therapies against biofilm infections and to contribute to the development of anti-infective medical devices. Figure 2. Bacterial biofilm formation and growth by S. epidermidis (ATCC 35984) exposed to natural Capsicum storage peptides (CSP). A) Growth (black bars) and biofilm (gray bars), both after 24 h of CSP solution exposure at 2 mg/ml. B) Bacterial growth kinetics of S. epidermidis exposed to CSP solution at 2 mg/ml (0, 1, 3, 6, 12 and 24 h). Untreated samples were used as growth and biofilm controls, and rifampicin (8 µg/mL) was used as an antibiotic control. The results were monitored by optical density (OD570 for biofilm formation and OD600 for bacteria growth), and untreated samples were considered 100% growth and biofilm formation. (*) represents significant differences when compared to biofilm/growth control using Student's t-test; p-values <0.05 were considered statistically significant. 4. CSP37 prediction and activity on biofilm formation and growth by S. epidermidis (ATCC 35984). A) Predicted peptides by MASCOT were identified using BLAST (NCBI) and the annotated Capsicum annuum proteome. This lead to the identification of two separated proteins (Capana01g000302 = XP_016559134.1 and Capana01g000309 = XP_016567465.1). B) Sequence alignment of the two proteins identified with BLAST. The two MASCOT are highlighted in black. The position of the two peptides in the protein sequences and the strong sequence similarity between these two proteins suggest that the two peptides can be in fact two consecutives portion of a unique protein of Capsicum baccatum. C) The mass of the CSP37 peptide (predicted from the union of the two precedent peptides) corresponds to the major MS pick of natural CSP fraction and, its 3D structural prediction by PEP-FOLD 3. D) Growth (black bars) and biofilm formation (gray bars) of S. epidermidis exposed to CSP37 (synthetic) solution at 0.23, 0.46 or 0.92 mM for 24 h. Untreated samples were used as growth and biofilm formation controls, and rifampicin (8 µg/mL) was used as an antibiotic control. The results were monitored by optical density (OD570 for biofilm formation and OD600 for bacteria growth), and untreated samples were considered 100% growth and biofilm formation. (*) represents significant differences when compared to biofilm/growth controls using Student's t-test; p-values <0.05 were considered statistically significant.

Declaration of interests

☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: R.G.V.B, S.C.B.G, A.R.Z, A.J.M and R.G. are co-authors of a patent register of CSP37. Application number EP19305205.7 at European Patent Office. Specific aspect of manuscript covered in patent application: its amino acid sequence and bioactivity.

Figure 1

 1 Figure 1

Figure 3

 3 Figure 3

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 1 .

 1 Figure 1. Identification of the peptides from C. baccatum active fraction. MALDI-MS mass spectra (linear, positive ion mode) of the antibiofilm active fraction peptides, range of 4,000-20,000 m/z. Table 1. (Insertion) Mascot results for CSP spots according to the NCBI CDD and BLAST searches using the proteome of C. annum as the database. (2SS protein) is 2S sulfur-rich seed storage protein; *For the Capsicum proteome database in Mascot, scores > 28 indicate identity or extensive homology (p<0.05).

Figure 3 .

 3 Figure 3. Scanning electron microscopy (SEM) images of Red pepper-coated slides after exposure to biofilm-forming S. epidermidis and the water contact angle (WCA) of Permanox™ slides before and after spin coating. A-B) Biofilm control: methanol (vehicle) was used to prepare untreated surface slides and anchored by spin coating three times. C-J) Red pepper coatings: natural capsicum storage peptides (CSP) were anchored by spin coating once or three times at 2 or 10 mg/ml. Scale bars: 500 and 2,000 x magnification. K) Graphic displaying the contact angle (degrees) results of noncoated and red pepper-coated Permanox™ slides. In the upper part, we show the photographs of the water drop and WCA, captured by a Theta Lite optical tensiometer and (Insertion) Spin Coat parameters. The WCAs are a mean of five measurements of different points of each sample.

Figure

  Figure4. CSP37 prediction and activity on biofilm formation and growth by S. epidermidis (ATCC 35984). A) Predicted peptides by MASCOT were identified using BLAST (NCBI) and the annotated Capsicum annuum proteome. This lead to the identification of two separated proteins (Capana01g000302 = XP_016559134.1 and Capana01g000309 = XP_016567465.1). B) Sequence alignment of the two proteins identified with BLAST. The two MASCOT are highlighted in black. The position of the two peptides in the protein sequences and the strong sequence similarity between these two proteins suggest that the two peptides can be in fact two consecutives portion of a unique protein of Capsicum baccatum. C) The mass of the CSP37 peptide (predicted from the union of the two precedent peptides) corresponds to the major MS pick of natural CSP fraction and, its 3D structural prediction by PEP-FOLD 3. D) Growth (black bars) and biofilm formation (gray bars) of S. epidermidis exposed to CSP37 (synthetic) solution at 0.23, 0.46 or 0.92 mM for 24 h. Untreated samples were used as growth and biofilm formation controls, and rifampicin (8 µg/mL) was used as an antibiotic control. The results were monitored by optical density (OD570 for biofilm formation and OD600 for bacteria growth), and untreated samples were considered 100% growth and biofilm formation. (*) represents significant differences when compared to biofilm/growth controls using Student's t-test; p-values <0.05 were considered statistically significant.

Figure 5 .

 5 Figure 5. Cytotoxicity of natural CSP and synthetic CSP37. A) Human lymphocytes cells after 24 h of exposure to Capsicum storage peptides (CSP) at 2.0 mg/ml, 1% Triton solution (cytotoxic control), or RPMI 1640 medium (untreated control). B) Hemolysis test with human erythrocytes exposed to CSP at 2 mg/ml, 1% Triton solution (100% of hemolysis), or phosphate buffered saline (PBS) (untreated control as nonhemolytic control). C-E) Dot plots of lymphocytes obtained by flow cytometry. The circled areas highlight the cell populations of interest: C) untreated control, D) cytotoxic control and E) cells exposed to CSP (2.0 mg/ml). (*) Represents a significant difference when compared to reference samples using ANOVA followed by Tukey's post hoc test; p-values <0.05 were considered statistically significant. F) CSP37 cytotoxicity evaluation in representative human cell lines at 25 µM. The number of normal cells is presented as the residual cell percentage (%) compared to the average of the control (DMSO, shown as white). The first three bars, which are black-gray, represent classic cytotoxic controls (roscovitine, doxycycline and Taxol), and the blue bar represents CSP37-exposed cells. Cells lines are described under the bars. An automated image-based cellular content analysis system (HCS/HCA) was used.

Figure 6 . 16 |

 616 Figure 6. Proposed mechanism of anti-adhesion and of biofilm formation inhibition by red pepper coating. The effects are due to repulsive forces between the polar material surface after CSP coating and the anionic S. epidermidis surface and biofilm matrix.

J. Name., 2013, 00, 1-3 | 3

J. Name., 2013, 00, 1-3 | 5

J. Name., 2013, 00, 1-3 | 11

J. Name., 2013, 00, 1-3 | 13

J. Name., 2013, 00, 1-3 | 15

Acknowledgements

We thankfully acknowledge the Centro de Microscopia Eletrônica -CME/UFRGS for assistance in electron microscopies, Fernanda Brum and Francine Santos for the support in the cytotoxicity and purification assays and Sharon Vieira do Reis for the assistance in the gel preparation.

MS and MALDI-TOF/TOF analysis; EG contributed to peptide prediction (bioinformatics analysis); MPB and KRZ contributed to biofilm analysis and RGVB contributed to all experimental tests and wrote the paper.

Funding

The authors declare that there is no conflict of interest. This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico -CNPq (process number 312351/2017-8 and 303353/2016-3), Fundação de Amparo à Pesquisa do Rio Grande do Sul -FAPERGS (PRONEM 16/2551-0000244-4), CAPES-COFECUB (88887.130212/2017-01), Instituto Nacional de Ciência e Tecnologia para Inovação Farmacêutica -INCT-if and the Graduate Program of Pharmaceutical Sciences -PPGCF/UFRGS and Fundação de Amparo à Pesquisa do Estado de São Paulo -FAPESP (process number 2009/54098-6; 2012/18031-), Brazil.

Conflicts of interest


come

Letter to the Editor Regarding the Article "Rotenone Increases Isoniazid Toxicity but Does Not Cause Significant Liver Injury: Implications for the Hypothesis that Inhibition of the Mitochondrial Electron Transport Chain Is a Common Mechanism of Idiosyncratic Drug-Induced Liver Injury" by Cho and Co-Workers, 2019 To the Editor: Idiosyncratic drug-induced liver injury (iDILI) is a major issue for the treated patients because of its unpredictability and its potential severity. It is also a concern for the pharmaceutical companies since severe or lethal liver injury can lead to the withdrawal of drugs from the market, or earlier during clinical trials, thus causing significant financial losses. [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF] While immune response plays a major role, [START_REF] Cho | How reactive metabolites induce an immune response that sometimes leads to an idiosyncratic drug reaction[END_REF] it is now accepted that mitochondrial dysfunction is also an important mechanism whereby drugs can induce iDILI. [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF][START_REF] Porceddu | Prediction of liver injury induced by chemicals in human with a multiparametric assay on isolated mouse liver mitochondria[END_REF][START_REF] Ramachandran | Mitochondrial dysfunction as a mechanism of drug-induced hepatotoxicity: current understanding and future perspectives[END_REF] The unpredictability of druginduced mitochondrial dysfunction in iDILI can be due to different causes such as drug-drug pharmacokinetic and/or pharmacodynamic interactions, the presence of an underlying liver disease, or different types of genetic predisposition affecting mitochondrial function. [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF][START_REF] Begriche | Drug-induced toxicity on mitochondria and lipid metabolism. Mechanistic diversity and deleterious consequences for the liver[END_REF] Genetic defects affecting the mitochondrial electron transport chain (mtETC) or the mitochondrial fatty acid oxidation (mtFAO) pathway have been reported to favor mitochondrial dysfunction and liver injury induced by several drugs such as the anticonvulsant drug valproic acid and some antiretroviral agents. [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF][START_REF] Begriche | Drug-induced toxicity on mitochondria and lipid metabolism. Mechanistic diversity and deleterious consequences for the liver[END_REF] In a recent article published in Chemical Research in Toxicology, Cho and co-workers treated C57BL/6 mice with rotenone (a prototypical inhibitor of the MRC complex I), isoniazid (an antituberculosis drug inhibiting the MRC complex II), or both compounds, over a six-week period. [START_REF] Cho | Rotenone increases isoniazid toxicity but does not cause significant liver injury: implications for the hypothesis that inhibition of the mitochondrial electron transport chain is a common mechanism of idiosyncratic drug-induced liver injury[END_REF] Whereas treatment with rotenone alone (0.05 or 0.1% w/w in food) or isoniazid alone (0.2% w/w in food) did not cause death among the animals, the coadministration of rotenone and isoniazid led to lethality in 100% of the mice. This toxicity was not related to liver injury as assessed by hepatic histology and serum glutamate dehydrogenase (GLDH) activity.

Notably, the latter investigations were performed after only 3 and 6 days since the cotreated mice did not survive afterwards. From these results, the authors conclude that inhibition of the mtETC is not a significant mechanism of iDILI. [START_REF] Cho | Rotenone increases isoniazid toxicity but does not cause significant liver injury: implications for the hypothesis that inhibition of the mitochondrial electron transport chain is a common mechanism of idiosyncratic drug-induced liver injury[END_REF] Although this in vivo study provides interesting data, this conclusion is somewhat misleading for the following reasons.

First and foremost, there is already strong evidence that mtETC impairment is responsible for liver injury with different drugs, especially with drugs altering the replication or translation of mitochondrial DNA (mtDNA), which encodes for 13 mtETC polypeptides. Of note, such alteration eventually leads to an impairment of electron transfer from both complexes I and II and onwards of the hepatic mtETC. For instance, phase II clinical trials with the anti-HBV drug fialuridine, which inhibits mtDNA replication, [START_REF] Fromenty | Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity[END_REF] were prematurely interrupted due to serious adverse effects including unmanageable lactic acidosis, microvesicular steatosis and liver failure requiring liver transplantation, or even leading to death. [START_REF] Mckenzie | Hepatic failure and lactic acidosis due to fialuridine (FIAU), an investigational nucleoside analogue for chronic hepatitis B[END_REF][START_REF] Mak | DNA polymerase inhibitors for treating hepatitis B: a safety evaluation[END_REF] Similar adverse effects can be induced by different antiretroviral nucleoside reverse-transcriptase inhibitors (NRTIs) such as zalcitabine (ddC), stavudine (d4T) and didanosine (ddI), 10,11 which also strongly inhibit mtDNA replication. [START_REF] Fromenty | Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity[END_REF]10 Experimental investigations with fialuridine and NRTIs demonstrated the major role of mtDNA depletion and mtETC impairment in the development of liver injury. 10,12,13 NRTI-induced hepatotoxicity occurs only in some patients, thus suggesting the role of underlying factors such as genetic predisposition. [START_REF] Begriche | Drug-induced toxicity on mitochondria and lipid metabolism. Mechanistic diversity and deleterious consequences for the liver[END_REF]14 Furthermore, other experimental investigations support the role of mtETC impairment in liver injury induced by amiodarone, 15,16 perhexiline 17 and buprenorphine. [START_REF] Berson | Mechanisms for experimental buprenorphine hepatotoxicity: major role of mitochondrial dysfunction versus metabolic activation[END_REF] Second, in the study of Cho and colleagues, the premature death from extra-hepatic causes of the mice cotreated with rotenone and isoniazid most probably precluded the possibility to observe delayed mitochondrial toxicity in liver and subsequent hepatic injury. [START_REF] Cho | Rotenone increases isoniazid toxicity but does not cause significant liver injury: implications for the hypothesis that inhibition of the mitochondrial electron transport chain is a common mechanism of idiosyncratic drug-induced liver injury[END_REF] Notably, liver is not the primary target organ during rotenone toxicity. [START_REF] Wood | Fatality after deliberate ingestion of the pesticide rotenone: a case report[END_REF] Human and animals studies showed that rotenone-induced mitochondrial toxicity causes respiratory depression, cardiovascular collapse and severe metabolic (i.e. lactic) acidosis. [START_REF] Wood | Fatality after deliberate ingestion of the pesticide rotenone: a case report[END_REF] In addition to heart and lungs, kidney and spleen are also damaged after rotenone exposure. [START_REF] Jiang | Rotenone induces nephrotoxicity in rats: oxidative damage and apoptosis[END_REF] Regarding isoniazid, although hepatotoxicity is a significant adverse event in treated patients, hypotension, renal failure and metabolic acidosis could also occur. [START_REF] Watkins | Isoniazid toxicity presenting as seizures and metabolic acidosis[END_REF][START_REF] Gokhale | Isoniazid toxicity presenting as status epilepticus and severe metabolic acidosis[END_REF] A thorough necropsy of the deceased mice might have uncovered which vital organs were severely damaged by rotenone. Moreover, in vitro investigations in hepatocytes might have permitted to evaluate the toxicity of rotenone, isoniazid and their combination. Indeed, previous investigations reported that rotenone and isoniazid could induce toxicity in hepatic cells. [START_REF] Isenberg | Role of the mitochondrial membrane permeability transition (MPT) in rotenone-induced apoptosis in liver cells[END_REF][START_REF] Mann | Evaluation of HepaRG cells for the assessment of indirect drug-induced hepatotoxicity using INH as a model substance[END_REF] Third, the authors looked for hepatic injury in liver sections by using hematoxylin and eosin staining. However, this staining method is not appropriate in order to detect microvesicular steatosis, [START_REF] Catta-Preta | A critical analysis of three quantitative methods of assessment of hepatic steatosis in liver biopsies[END_REF] a liver lesion commonly occurring with drugs and toxins inducing mitochondrial dysfunction. [START_REF] Fromenty | Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity[END_REF][START_REF] Massart | Drug-induced inhibition of mitochondrial fatty acid oxidation and steatosis[END_REF] Regardless of the pathophysiological context, minor or moderate microvesicular steatosis is better detected with Oil red O or Sudan III staining on frozen liver section. [START_REF] Catta-Preta | A critical analysis of three quantitative methods of assessment of hepatic steatosis in liver biopsies[END_REF][START_REF] Tandra | Presence and significance of microvesicular steatosis in nonalcoholic fatty liver disease[END_REF] Investigations in animals are useful in order to study DILI including iDILI, but different factors may greatly modulate the severity of liver injury such as drug distribution and metabolism as well as animal species and strain. [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF][START_REF] Amacher | The primary role of hepatic metabolism in idiosyncratic druginduced liver injury[END_REF][START_REF] Mcgill | Animal models of drug-induced liver injury[END_REF] Interestingly, isoniazid did not induce microvesicular steatosis in C57BL/6J mice, whereas this liver lesion was observed in other strains such as BALB/cJ, DBA/2J and LG/J mice. [START_REF] Church | A systems biology approach utilizing a mouse diversity panel identifies genetic differences influencing isoniazid-induced microvesicular steatosis[END_REF] The latter study and others [START_REF] Labbe | Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies[END_REF]14,[START_REF] Stewart | Polymerase γ gene POLG determines the risk of sodium valproate-induced liver toxicity[END_REF][START_REF] Lucena | Mitochondrial superoxide dismutase and glutathione peroxidase in idiosyncratic drug-induced liver injury[END_REF] underline the importance of genetic susceptibility in the occurrence of drug-induced liver injury, in particular when considering genes encoding for mitochondrial proteins. Importantly, druginduced mtETC toxicity can cause not only microvesicular steatosis and cell death but also other liver lesions such as steatohepatitis and cholestasis. [START_REF] Begriche | Drug-induced toxicity on mitochondria and lipid metabolism. Mechanistic diversity and deleterious consequences for the liver[END_REF]17,[START_REF] Borgne-Sanchez | Mitochondrial dysfunction in druginduced liver injury[END_REF] Hence, more investigations are clearly needed in order to identify the main factors able to modulate drug-induced mitochondrial dysfunction in liver. While in vivo investigations may better reflect the complexity of iDILI, complementary in vitro experiments are also useful to decipher the mechanisms whereby drugs can be toxic for mitochondria.

Bernard Fromenty

  de Rennes, INRAE, Nutrition, Metabolisms, and Cancer (NuMeCan) Institut, UMR_A 1341, UMR_S 1241 , F-35000 Rennes , France.

  INSERM, Univ Rennes, INRA, Institut NUMECAN (Nutrition Metabolisms and Cancer) UMR_A 1341, UMR_S 1241, F-35000 Rennes, France (10) Igoudjil, A., Begriche, K., Pessayre, D., and Fromenty, B. (2006) Mitochondrial, metabolic and genotoxic effects of antiretroviral nucleoside reverse-transcriptase inhibitors. Anti-Infect. Agents Med. Chem. 5, 273-292. (11) Wang, Y., Lin, Z., Liu, Z., Harris, S., Kelly, R., Zhang, J., Ge, W., Chen, M., Borlak, J., and Tong, W. (2013) A unifying ontology to integrate histological and clinical observations for drug-induced liver injury. Am. J. Pathol. 182, 1180-1187. (12) Lewis, W., Griniuviene, B., Tankersley, K.O., Levine, E.S., Montione, R., Engelman, L., de Courten-Myers, G., Ascenzi, M.A., Hornbuckle, W.E., Gerin, J.L., and Tennant, B.C. (1997) Depletion of mitochondrial DNA, destruction of mitochondria, and accumulation of lipid droplets result from fialuridine treatment in woodchucks (Marmota monax). Lab. Invest. 76, 77-87. (13) Lebrecht, D., Vargas-Infante, Y.A., Setzer, B., Kirschner, J., and Walker, U.A. (2007) Uridine supplementation antagonizes zalcitabine-induced microvesicular steatohepatitis in mice. Hepatology 45, 72-79. (14) Bailey, C.M., Kasiviswanathan, R., Copeland, W.C., and Anderson, K.S. (2009) R964C mutation of DNA polymerase gamma imparts increased stavudine toxicity by decreasing nucleoside analog discrimination and impairing polymerase activity. Antimicrob. Agents Chemother. 53, 2610-2612. (15) Fromenty, B., Fisch, C., Berson, A., Lettéron, P., Larrey, D., and Pessayre, D. (1990) Dual effect of amiodarone on mitochondrial respiration. Initial protonophoric uncoupling effect followed by inhibition of the respiratory chain at the levels of complex I and complex II. J. Pharmacol. Exp. Ther. 255, 1377-1384. (16) Felser, A., Blum, K., Lindinger, P.W., Bouitbir, J., and Krähenbühl, S. (2013) Mechanisms of hepatocellular toxicity associated with dronedarone-a comparison to amiodarone. Toxicol. Sci. 131, 480-490. (17) Deschamps, D., DeBeco, V., Fisch, C., Fromenty, B., Guillouzo, A., and Pessayre, D. (1994) Inhibition by perhexiline of oxidative phosphorylation and the beta-oxidation of fatty acids: possible role in pseudoalcoholic liver lesions. Hepatology 19, 948-961.

ACKNOWLEDGEMENTS

I am very grateful to Dr Julie Massart for her critical reading of this letter.

AUTHOR INFORMATION

ORCID

Bernard Fromenty: 0000-0003-2994-5465