Comparative Analysis of Listeria monocytogenes Plasmids and Expression Levels of Plasmid-Encoded Genes during Growth under Salt and Acid Stress Conditions.

Patricia Hingston,Siyun Wang,Lisbeth Truelstrup Hansen,Thomas Brenner

doi:10.3390/toxins11070426

Patricia Hingston, Siyun Wang + Show 2 more

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https://doi.org/10.3390/toxins11070426

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Abstract

Listeria monocytogenes strains are known to harbour plasmids that confer resistance to sanitizers, heavy metals, and antibiotics; however, very little research has been conducted into how plasmids may influence L. monocytogenes’ ability to tolerate food-related stresses. To investigate this, a library (n = 93) of L. monocytogenes plasmid sequences were compared. Plasmid sequences were divided into two groups (G1 and G2) based on a repA phylogeny. Twenty-six unique plasmid types were observed, with 13 belonging to each of the two repA-based groups. G1 plasmids were significantly (p < 0.05) smaller than G2 plasmids but contained a larger diversity of genes. The most prevalent G1 plasmid (57,083 bp) was observed in 26 strains from both Switzerland and Canada and a variety of serotypes. Quantitative PCR (qPCR) revealed a >2-fold induction of plasmid-contained genes encoding an NADH peroxidase, cadmium ATPase, multicopper oxidase, and a ClpL chaperone protein during growth under salt (6% NaCl) and acid conditions (pH 5) and ProW, an osmolyte transporter, under salt stress conditions. No differences in salt and acid tolerance were observed between plasmid-cured and wildtype strains. This work highlights the abundance of specific plasmid types among food-related L. monocytogenes strains, the unique characteristics of G1 and G2 plasmids, and the possible contributions of plasmids to L. monocytogenes tolerance to food-related stresses.

Highlights

Listeria monocytogenes is a ubiquitous, facultatively anaerobic, foodborne pathogen that is capable of growing at temperatures as low as −0.4 ◦ C [1], at remarkably low pH [2], and in high osmolarity environments [3], making it dangerous as a foodborne pathogen; especially with regards to refrigerated, ready-to-eat foods
In a previous study [30], we found that L. monocytogenes strains possessing a plasmid had significantly faster growth rates in acidified media compared to plasmid-free strains, prompting the hypothesis that genes found in L. monocytogenes plasmids contribute to acid tolerance
Data points denote the averages of the three replicates and error bars represent standard deviations. This study examined both the genetic similarities and differences between two L. monocytogenes repA

Summary

Introduction

Listeria monocytogenes is a ubiquitous, facultatively anaerobic, foodborne pathogen that is capable of growing at temperatures as low as −0.4 ◦ C [1], at remarkably low pH [2], and in high osmolarity environments [3], making it dangerous as a foodborne pathogen; especially with regards to refrigerated, ready-to-eat foods. Plasmid harbourage rates have been shown to be higher amongst reoccurring strains of L. monocytogenes (75%) as opposed to sporadic strains (35%) isolated from food or food-processing facilities [20], suggesting that genes found in L. monocytogenes plasmids may be beneficial for the survival of this pathogen in such environments. L. monocytogenes isolates containing the plasmid-encoded bcrABC genes quadruple the MIC of benzalkonium chloride, a common ammonium-based food industry disinfectant, compared to plasmid cured strains [11]. Frequent application of such disinfectants in the food industry may create a selective pressure favouring the uptake of brcABC containing plasmids from other L. monocytogenes strains and lead to a higher prevalence among food-related isolates

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