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 ΔRD30 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.