Comprehensive phenotypic and genomic study for understanding the variability in susceptibility of Listeria monocytogenes to AMPs and bacteriocins

In this study, we investigated the inhibitory activity of six antimicrobial peptides against 40 Listeria monocytogenes isolates originating from clinical, environmental, or food matrices. The antibiotic susceptibility assays on the panel revealed high resistance to clindamycin and lower resistance levels to ciprofloxacin. In addition, four isolates were shown to be resistant to at least one or two antibiotics. Virulome analysis revealed that the prfA-virulence gene cluster, the main pathogenicity determinant marker in Listeria monocytogenes (prfA, plcA, plcB, hly, mpl and actA) was detected in selected strains (n=15). The anti-listeria activities of the bacteriocins, bactofencin A (M14L, M18L), nisin Z, pediocin PA-1 (M31L) and plantaricin S, as well as two lipopeptides, brevibacillin and its synthetic analog brevibacillin Thr1 were tested. These molecules were selected based on their diversity in term of structures and mechanisms of action. The minimal inhibitory (MIC) and bactericidal concentrations (MBC) indicate that all peptides were actives against all the Listeria isolates with MIC values ranging between 0.54 uM and 18.20 uM. In general, the antimicrobial activity of the selected peptides was neither associated to the antibiotic resistance profile, nor to the presence/absence of virulence genes. In order to further understand the heterogeneity patterns of antimicrobial activity of the antimicrobial peptides through the selected isolates, a comparative genomic and structural impact analysis was carried out. The results revealed polymorphisms occurring in several genes associated with stress response, efflux pumps, membrane and protein biosynthesis. These polymorphisms could help to explain the decreased susceptibility profiles of certain isolates to the tested peptides. Overall, through their diversity in structure and mechanism of action, these peptides have been showed to be powerful inhibitors of Listeria monocytogenes and could serve as alternative to conventional antibiotics for the control of this pathogen in several environments.