Genotypes Associated with Listeria monocytogenes Isolates Displaying Impaired or Enhanced Tolerances to Cold, Salt, Acid, or Desiccation Stress

Abstract

The human pathogen <i>Listeria monocytogenes</i> is a large concern in the food industry where its continuous detection in food products has caused a string of recalls in North America and Europe. Most recognized for its ability to grow in foods during refrigerated storage, <i>L. monocytogenes</i> can also tolerate several other food-related stresses with some strains possessing higher levels of tolerances than others. The objective of this study was to use a combination of phenotypic analyses and whole genome sequencing to elucidate potential relationships between <i>L. monocytogenes</i> genotypes and food-related stress tolerance phenotypes. To accomplish this, 166 <i>L. monocytogenes</i> isolates were sequenced and evaluated for their ability to grow in cold (4°C), salt (6% NaCl, 25°C), and acid (pH 5, 25°C) stress conditions as well as survive desiccation (33% RH, 20°C). The results revealed that the stress tolerance of <i>L. monocytogenes</i> is associated with serotype, clonal complex (CC), full length <i>inlA</i> profiles, and the presence of a plasmid which was identified in 55% of isolates. Isolates with full length <i>inlA</i> exhibited significantly (<i>p</i> < 0.001) enhanced cold tolerance relative to those harboring a premature stop codon (PMSC) in this gene. Similarly, isolates possessing a plasmid demonstrated significantly (<i>p</i> = 0.013) enhanced acid tolerance. We also identified nine new <i>L. monocytogenes</i> sequence types, a new <i>inlA</i> PMSC, and several connections between CCs and the presence/absence or variations of specific genetic elements. A whole genome single-nucleotide-variants phylogeny revealed sporadic distribution of tolerant isolates and closely related sensitive and tolerant isolates, highlighting that minor genetic differences can influence the stress tolerance of <i>L. monocytogenes</i>. Specifically, a number of cold and desiccation sensitive isolates contained PMSCs in σ^B regulator genes (<i>rsbS, rsbU, rsbV</i>). Collectively, the results suggest that knowing the sequence type of an isolate in addition to screening for the presence of full-length <i>inlA</i> and a plasmid, could help food processors and food agency investigators determine why certain isolates might be persisting in a food processing environment. Additionally, increased sequencing of <i>L. monocytogenes</i> isolates in combination with stress tolerance profiling, will enhance the ability to identify genetic elements associated with higher risk strains.