Genome Analysis of Listeria monocytogenes Sequence Type 8 Strains Persisting in Salmon and Poultry Processing Environments and Comparison with Related Strains.

Annette Fagerlund,Trond Møretrø,Bjørn C T Schirmer,Even Heir,Solveig Langsrud,Ivo G Boneca

doi:10.1371/journal.pone.0151117

Abstract

Listeria monocytogenes is an important foodborne pathogen responsible for the disease listeriosis, and can be found throughout the environment, in many foods and in food processing facilities. The main cause of listeriosis is consumption of food contaminated from sources in food processing environments. Persistence in food processing facilities has previously been shown for the L. monocytogenes sequence type (ST) 8 subtype. In the current study, five ST8 strains were subjected to whole-genome sequencing and compared with five additionally available ST8 genomes, allowing comparison of strains from salmon, poultry and cheese industry, in addition to a human clinical isolate. Genome-wide analysis of single-nucleotide polymorphisms (SNPs) confirmed that almost identical strains were detected in a Danish salmon processing plant in 1996 and in a Norwegian salmon processing plant in 2001 and 2011. Furthermore, we show that L. monocytogenes ST8 was likely to have been transferred between two poultry processing plants as a result of relocation of processing equipment. The SNP data were used to infer the phylogeny of the ST8 strains, separating them into two main genetic groups. Within each group, the plasmid and prophage content was almost entirely conserved, but between groups, these sequences showed strong divergence. The accessory genome of the ST8 strains harbored genetic elements which could be involved in rendering the ST8 strains resilient to incoming mobile genetic elements. These included two restriction-modification loci, one of which was predicted to show phase variable recognition sequence specificity through site-specific domain shuffling. Analysis indicated that the ST8 strains harbor all important known L. monocytogenes virulence factors, and ST8 strains are commonly identified as the causative agents of invasive listeriosis. Therefore, the persistence of this L. monocytogenes subtype in food processing facilities poses a significant concern for food safety.

Highlights

Listeria monocytogenes is an opportunistic Gram-positive pathogen responsible for the foodborne disease listeriosis
Repeated isolation during a timeframe of 13 years of the same molecular subtype of L. monocytogenes in the same salmon processing facility indicated that this subtype has a good ability to persist in salmon processing environments, either in the processing facility itself or another environment that serves as a source for reintroduction
The observed divergence in each group was comparable to that observed in several studies investigating L. monocytogenes outbreaks: In the study by Orsi et al [9], four L. monocytogenes strains derived from a clone persisting in a food processing facility over 12 years differed by 11 single-nucleotide polymorphisms (SNPs) outside of the comK prophage region, while Gilmour et al [8] showed that 28 SNPs differentiated two clinical isolates from an outbreak of listeriosis

Summary

Introduction

Listeria monocytogenes is an opportunistic Gram-positive pathogen responsible for the foodborne disease listeriosis. It has a saprophytic lifestyle and is ubiquitously present in the environment, including in water, soil and vegetation, and can survive in food products and in food processing plants. Persistence is often defined as the recurrent isolation of the same molecular subtype in the same food processing plant over a long time period [2,3,4]. It should be noted that the ability to differentiate between a potentially resident clonal strain and repeated introduction of similar but non-clonal strains from the outside environment will depend on the discriminatory power of the subtyping method employed [2]

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Discussion

Conclusion