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Summary Whole genome sequencing is increasing used in epidemiology, e.g. for tracing outbreaks of food‐borne diseases. This requires in‐depth understanding of pathogen emergence, persistence and genomic diversity along the food production chain including in food processing plants. We sequenced the genomes of 80 isolates of Listeria monocytogenes sampled from Danish food processing plants over a time‐period of 20 years, and analysed the sequences together with 10 public available reference genomes to advance our understanding of interplant and intraplant genomic diversity of L. monocytogenes. Except for three persisting sequence types (ST) based on Multi Locus Sequence Typing being ST7, ST8 and ST121, long‐term persistence of clonal groups was limited, and new clones were introduced continuously, potentially from raw materials. No particular gene could be linked to the persistence phenotype. Using time‐based phylogenetic analyses of the persistent STs, we estimate the L. monocytogenes evolutionary rate to be 0.18–0.35 single nucleotide polymorphisms/year, suggesting that the persistent STs emerged approximately 100 years ago, which correlates with the onset of industrialization and globalization of the food market.

Listeria monocytogenes can cause the serious infection listeriosis, which despite antibiotic treatment has a high mortality. Understanding the response of L. monocytogenes to antibiotic exposure is therefore important to ensure treatment success. Some bacteria survive antibiotic treatment by formation of persisters, which are a dormant antibiotic-tolerant subpopulation. The purpose of this study was to determine whether L. monocytogenes can form persisters and how bacterial physiology affects the number of persisters in the population. A stationary-phase culture of L. monocytogenes was adjusted to 10 8 CFU ml ؊1, and 10 3 to 10 4 CFU ml ؊1 survived 72-h treatment with 100 g of norfloxacin ml ؊1 , indicating a persister subpopulation. This survival was not caused by antibiotic resistance as regrown persisters were as sensitive to norfloxacin as the parental strain. Higher numbers of persisters (10 5 to 10 6 ) were surviving when older stationary phase or surface-associated cells were treated with 100 g of norfloxacin ml ؊1. The number of persisters was similar when a ⌬sigB mutant and the wild type were treated with norfloxacin, but the killing rate was higher in the ⌬sigB mutant. Dormant norfloxacin persisters could be activated by the addition of fermentable carbohydrates and subsequently killed by gentamicin; however, a stable surviving subpopulation of 10 3 CFU ml ؊1 remained. Nitrofurantoin that has a growth-independent mode of action was effective against both growing and dormant cells, suggesting that eradication of persisters is possible. Our study adds L. monocytogenes to the list of bacterial species capable of surviving bactericidal antibiotics in a dormant stage, and this persister phenomenon should be borne in mind when developing treatment regimens.

Aims: To study the importance of bacteriocin production for the antilisterial effect of a bacteriocinogenic Carnobacterium piscicola strain A9b on growth of Listeria monocytogenes in broth and cold-smoked salmon systems. Methods and Results: Acriflavin treatment of strain A9b resulted in loss of bacteriocin production and of immunity to carnobacteriocin B2. Two plasmids present in the wild-type were lost in the variant that was also more sensitive to bavaricin and leucocin A than the wild-type indicating cross-resistance to class IIa bacteriocins. The growth rate of the bac ) mutant was higher than that of the wild-type at 5 and 37°C but not at 25 or 30°C. In salmon juice the maximum cell density of L. monocytogenes was suppressed 3 and 6 log by co-culture with C. piscicola A9b bac ) and bac + , respectively, as compared with the control. Sterile filtered cultures of C. piscicola A9b bac ) caused a limited suppression of the maximum cell density of L. monocytogenes similar to that observed when sterile buffer was added in equal amounts. Semi-purified carnobacteriocin B2 caused a 3AE5 log decline in viable cell count after 6 day of incubation in cold-smoked salmon juice at 5°C. High resistance level to carnobacteriocin B2 was observed for L. monocytogenes cells exposed to semi-purified and in situ produced carnobacteriocin B2. Conclusions:The presence of bacteriocin production in C. piscicola enhances its inhibition of L. monocytogenes Significance and Impact of the Study: Due to the emergence of resistance, a bacteriocin negative lactic acid bacteria may be more suited for practical use as a bioprotective agent against L. monocytogenes in ready-to-eat foods.

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