Contributions to selected phenotypic characteristics of large species- and lineage-specific genomic regions in Listeria monocytogenes.

We hypothesized that genomic regions specific to Listeria monocytogenes or selected L. monocytogenes strains may contribute to virulence and phenotypic differences among the strains. A whole genome alignment of two completed L. monocytogenes genomes and the one completed Listeria innocua genome initially identified 28 genomic regions of difference (RD) > 4 kb that were found in one or both L. monocytogenes genomes, but absent from the non-pathogenic L. innocua. In silico analyses using an additional 18 draft L. monocytogenes genomes showed that (i) 15 RDs were found in all or most L. monocytogenes genomes; (ii) three RDs were found in all or most lineage I genomes, but absent from lineage II genomes; and (iii) four RDs were found in all lineage II genomes, but no lineage I genomes. Null mutants in two L. monocytogenes-specific RDs (RD16 and RD30; found in most L. monocytogenes) and the lineage II-specific RD25 showed no evidence for impaired invasion or intracellular growth in selected tissue culture cells. Although, in pH 5.5 minimal media, the DeltaRD30 null mutant showed reduced ability to compete with its parent strain, indicating that RD30 may have a role in L. monocytogenes growth under limited nutrient conditions at acidic pH.

Growth and persistence of Listeria monocytogenes isolates on the plant model Arabidopsis thaliana.

While the majority of human listeriosis cases appear to be linked to consumption of processed ready-to-eat foods (e.g., deli meats), a few listeriosis outbreaks have been linked to consumption of contaminated vegetables. In this study, we assessed four isolates representing the major Listeria monocytogenes lineages for their abilities to attach to and grow on Arabidopsis thaliana, a well-characterized plant model. When plants were dipped for 5min into 3ml of water containing 8.8logCFU of L. monocytogenes and rinsed repeatedly, L. monocytogenes was recovered from the leaves at densities from 1.52 to 2.17logCFU/cm(2). Ten days after exposure, bacterial numbers had increased over initial numbers by 2.60-2.95logCFU/cm(2). Using L. monocytogenes expressing GFP, bacteria were visualized in the intercellular spaces of A. thaliana leaves, suggesting internalization through stomata. These data indicate that L. monocytogenes can rapidly attach to and multiply on plant surfaces and colonize intercellular spaces in A. thaliana leaves where it may be protected from sanitation treatments. When A. thaliana seeds were exposed to L. monocytogenes, between 4.23 and 4.57logCFU/cm(2) were recovered from leaves 7 days post-germination, suggesting that contaminated seeds can produce contaminated plants. Overall, our study demonstrates that prevention of L. monocytogenes contamination of plants throughout growing stages is critical, consistent with recommendations for other produce-transmitted foodborne pathogens.