Abstract
The aim of this study was to compare the phenotype of 40 strains of L. monocytogenes under food and host relevant stress conditions. The strains were chosen to represent food and clinical isolates and to be equally distributed between the most relevant clonal complexes for clinical and food isolates (CC1 and CC6 vs CC121 and CC9), plus one group of eight strains of rare clonal complexes. Human-associated CC1 had a faster maximal growth rate than the other major complexes, and the lag time of CC1 and CC6 was significantly less affected by the addition of 4% NaCl to the medium. Food-associated CC9 strains were hypohemolytic compared to other clonal complexes, and all strains found to be resistant to increased concentrations of benzalkonium chloride belonged to CC121 and were positive for Tn6188 carrying the qacH gene. Lactic acid affected the survival of L. monocytogenes more than HCl, and there was a distinct, strain specific pattern of acid tolerant and sensitive strains. Strains from CC6 and human clinical isolates are less resilient under acid stress than those from other complexes and from food. One strain isolated from a human patient exhibited significant growth defects across all conditions.
Highlights
The severe clinical conditions resulting from food-borne infections with Listeria monocytogenes are a constant concern to public health agencies and food producers
The highest Vmax was observed in strain N12-1996 (CC1, serotype 4b, food isolate; 8.1, SD = 0.5), the slowest Vmax was in strain N12-1387 (CC6, serotype 4b, human clinical isolate; 3.86, SD = 0.05)
We found that lineage I strains recover faster after exposure to 4% NaCl in the medium and CC1 had a higher Vmax compared to CC121 and CC9 in both brain heart infusion (BHI) supplemented with NaCl and BHI
Summary
The severe clinical conditions resulting from food-borne infections with Listeria monocytogenes are a constant concern to public health agencies and food producers. Maury et al.[16] analyzed the relative prevalence of different clonal complexes (CC) in a large dataset of 6633 L. monocytogenes strains isolated from human clinical cases and food. CC121 or CC9 strains were found more frequently in patients with clinical comorbidities[16] This may explain why an infrequent number of strains with mutations in important virulence factors are found in human patients[6,17,18]. We characterized a set of 40 L. monocytogenes isolates for their response to food associated stresses (preservatives like salt and lactic acid, and the antimicrobial benzalkonium chloride (BC)) as well as attributes relevant in the host environment (resistance to HCl, hemolytic activity as an approximation for the expression rate of virulence genes). The strain set was chosen to be distributed between food and clinical isolates as well as CC1, CC6, CC121 and CC9, plus an additional set of nine strains to represent rare isolates not commonly found in clinical cases or the food environment
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