Some Aspects of Resistance Development against Nisin and Human Neutrophil Peptide-1 in Enterococcus faecalis

Augmented resistance against antimicrobial peptides in pathogenic bacteria has become a serious concern leading to increased bacterial virulence, which limits the applications of these peptides as biopreservatives. Nisin and Human Neutrophil Peptide-1 (HNP-1) are the peptides with biopreservative/clinical and physiological relevance, respectively. In the present study, certain aspects of resistance development against these peptides were investigated in Enterococcus faecalis . Three strains of E. faecalis with different levels of resistance against nisin were selected. Also, wild-type bacteria were challenged with low and high doses of HNP-1. Using in silico analysis, we identified a two-component cationic antimicrobial peptide (CAMP) sensing system in E. faecalis . Gene expression analysis revealed this system to be activated in nisin-resistant variants with increased net positive charge on bacterial cell surface. Cytochrome c assay and thin layer chromatography of the lipids derived from these bacterial strains corroborate increased cell surface positive charge upon resistance acquisition. The identified sensing system was not found to be activated in HNP-1-challenged cells, although an increased positive charge was found on the surface of these cells, indicating the possibility of more than one CAMP sensing system in E. faecalis . Both cell surface hydrophobicity and biofilm formation were increased in nisin-resistant strains, although biofilm formation was found to remain unaffected in HNP-1 challenged cells, which might be related to their unaltered expression levels of dltA gene.