[Characterization of a naturally atypical Listeria monocytogenes strain lacking inlAB]

We attempted to gain insights into the phenotypic and genetic characteristics of a naturally atypical Listeria monocytogenes strain S10 lacking inlAB.

The isolate S10, together with 7 L. monocytogenes strains representing different serovars, were studied by biochemical profiling, adhesion assay in HeLa cells, virulence to mice, detection of virulence-associated genes detection and genetic lineage analyses.

The S10 isolate belonged to lineage I and serovar 1/2b, with carbohydrate fermentation and hemolytic characteristics typical of L. monocytogenes. However, this strain had reduced adhesion to HeLa cells and decreased pathogenicity to mice. It lacked the gene locus inlAB and their adjacent genes lmo0431, lmo0432, lmo0436 and lmo0437, but contained an almost whole set of infection-associated genes. S10 fell into the lineage I cluster and form a sister branch with 4b strain.

The isolate S10 represents the first report of atypical L. monocytogenes lacking inlAB. As it lacked the inlAB locus from an otherwise typical lineage I genetic background, inlAB was probably lost from the genome via independent deletion event.