Phenotypic and phylogenetic analyses of Listeria monocytogenes strains reveal enhanced bile tolerance in clinical isolates

Introduction. Molecular epidemiological and phenotypic analyses of Listeria monocytogenes strains can inform our understanding of factors that influence onward transmission, virulence potential and ability to control the pathogen in foods or in clinical settings. Bile acids represent a host-specific barrier to microbial colonization of the gastrointestinal tract and are, therefore, not encountered in the external environment. We tested the hypothesis that tolerance of bile acids may be an evolutionary adaptation across L. monocytogenes clonal complexes (CCs), which varies with genotype and/or is associated with clinical isolates. Hypothesis. We hypothesized that strains of L. monocytogenes may differ in bile tolerance (a potential virulence-associated trait) and herein examine this phenomenon according to CC, strain origin and genotype. Aim. To assess 205 genome-sequenced isolates of L. monocytogenes for tolerance of porcine bile acids and bile salt hydrolase (BSH) activity. Methodology. Survival of L. monocytogenes strains was determined following exposure to porcine bile acids under conditions that mimic the small intestinal environment. BSH activity was assayed against pure taurodeoxycholic acid and glycodeoxycholic acid using an agar plate deconjugation assay. Genomes were analysed for polymorphisms in known bile tolerance loci. Results. Isolates demonstrated distinct inter-individual variances in bile tolerance under anaerobic conditions that mimic the intestinal environment. Strains isolated from cases of human disease were significantly more bile-tolerant than those isolated from natural environments or foods. There was no correlation between levels of bile tolerance and the size of bile precipitation zones on the BSH agar plate assay. No significant patterns were seen upon analysis of known or putative bile tolerance loci; however, individual strains with naturally occurring sigB operon mutations demonstrated reduced bile tolerance. Conclusion. L. monocytogenes strains isolated from clinical cases of listeriosis demonstrated elevated bile resistance consistent with a likely enhanced capacity to cause gastrointestinal infection preceding invasive disease. The data suggest the potential importance of bile tolerance in L. monocytogenes infection and highlight underlying molecular mechanisms by which strains vary in their natural levels of bile tolerance.