Abstract
The food-borne pathogen Listeria monocytogenes is known for its capacity to cope with multiple stress conditions occurring in food and food production environments (FPEs). Plasmids can provide benefits to their host strains, and it is known that various Listeria strains contain plasmids. However, the current understanding of plasmid frequency and function in L. monocytogenes strains remains rather limited. To determine the presence of plasmids among L. monocytogenes strains and their potential contribution to stress survival, a comprehensive dataset was established based on 1,921 published genomes from strains representing 14 L. monocytogenes sequence types (STs). Our results show that an average of 54% of all L. monocytogenes strains in the dataset contained a putative plasmid. The presence of plasmids was highly variable between different STs. While some STs, such as ST1, ST2, and ST4, contained few plasmid-bearing strains (<15% of the strains per ST), other STs, such as ST121, ST5, ST8, ST3, and ST204, possessed a higher proportion of plasmid-bearing strains with plasmids found in >71% of the strains within each ST. Overall, the sizes of plasmids analyzed in this study ranged from 4 to 170 kbp with a median plasmid size of 61 kbp. We also identified two novel groups of putative Listeria plasmids based on the amino acid sequences of the plasmid replication protein, RepA. We show that highly conserved plasmids are shared among Listeria strains which have been isolated from around the world over the last few decades. To investigate the potential roles of plasmids, nine genes related to stress-response were selected for an assessment of their abundance and conservation among L. monocytogenes plasmids. The results demonstrated that these plasmid genes exhibited high sequence conservation but that their presence in plasmids was highly variable. Additionally, we identified a novel transposon, Tn7075, predicted to be involved in mercury-resistance. Here, we provide the largest plasmid survey of L. monocytogenes to date with a comprehensive examination of the distribution of plasmids among L. monocytogenes strains. Our results significantly increase our knowledge about the distribution, composition, and conservation of L. monocytogenes plasmids and suggest that plasmids are likely important for the survival of L. monocytogenes in food and FPEs.
Highlights
The food-borne pathogen Listeria monocytogenes is a considerable public health concern because of its complex ecology involving environmental saprophytic, food-production environment-associated, and intracellular life cycles (Freitag et al, 2009)
We present a large and representative dataset consisting of 1,921 L. monocytogenes genomes, including strains of 14 sequence types (STs) collected from 32 countries over a period of 60 years
This is by far the most extensive dataset used for a plasmid survey in L. monocytogenes
Summary
The food-borne pathogen Listeria monocytogenes is a considerable public health concern because of its complex ecology involving environmental saprophytic, food-production environment-associated, and intracellular life cycles (Freitag et al, 2009). L. monocytogenes can tolerate a number of food-relevant stress conditions, including high salt concentrations and acidic pH, and is capable of growing at refrigeration temperatures (Bucur et al, 2018). Because of these stress survival capabilities, L. monocytogenes can persist for extended time periods in FPEs despite the regular rigorous cleaning procedures used to mitigate microbial contamination (Carpentier and Cerf, 2011; Ferreira et al, 2014). The persistence and survival of L. monocytogenes in FPEs represents a considerable challenge for food safety as it provides opportunities for L. monocytogenes to contaminate food. Many listeriosis outbreaks have been linked to L. monocytogenes contamination of RTE foods including as deli meat, cheeses, fruits, and vegetables (Garner and Kathariou, 2016; Buchanan et al, 2017; Lopez-Valladares et al, 2018)
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