Listeria monocytogenes is a causative agent of foodborne infection in humans and animals. The virulence factor InlB interacts with mammalian receptor c-Met via its internalin domain to provide L. monocytogenes invasion in non-professional phagocytes. Naturally occurring InlB internalin domain variants form four subclusters on the maximal likelihood tree. Four variants belonging to distinct subclusters were cloned into the vector carrying 3΄ and 5΄-flanking sequences to restore full length inlB and expressed in the L. monocytogenes strain EGDeΔinlB. The substitutions Val132Ile, Thr117Ala and Ile138Leu, Thr251Met/Ser were specific for variants 13, 14 and 1, respectively, the variant 9 carried Ser73Asn, Ile91Val, Leu164Pro, Met251Ser/Thr substitutions. All InlB variants improved invasion of the parental strain in murine colon carcinoma C26 cells with 4.6-fold difference between the most and least effective variants (variants 14 and 13, respectively, P < 0.05). Bacterial loads in livers of intragastrically infected mice were 258, 149 and 92 times higher for variant 14, 13 and 1 carrying strains, respectively, than for EGDeΔinlB (P < 0.01). In contrast, the variant 9 did not noticeably improve infection comparatively to the parental strain. Overall, obtained results demonstrated that naturally occurred InlB internalin domain variants differed in their ability to support intragastric infection in mice.

The pathogenic Gram-positive bacterium Listeria monocytogenes has been evolving into a few phylogenetic lineages. Phylogenetically defined substitutions were described in the L. monocytogenes virulence factor InlB, which mediates active invasion into mammalian cells via interactions with surface receptors c-Met and gC1q-R. InlB internalin domain (idInlB) is central to interactions with c-Met. Here we compared activity of purified recombinant idInlB isoforms characteristic for L. monocytogenes phylogenetic lineage I and II. Size exclusion chromatography and intrinsic fluorescence were used to characterize idInlBs. Western blotting was used to study activation of c-Met-dependent MAPK- and PI3K/Akt-pathways. Solid-phase microplate binding and competition assay was used to quantify interactions with gCq1-R. Isogenic recombinant L. monocytogenes strains were used to elucidate the input of idInlB isoforms in HEp-2 cell invasion. Physicochemical parameters of idInlB isoforms were similar but not identical. Kinetics of Erk1/2 and Akt phosphorylation in response to purified idInlBs was lineage specific. Lineage I but not lineage II idInlB specifically bound gC1q-R. Antibody against gC1q-R amino acids 221–249 inhibited invasion of L. monocytogenes carrying lineage I but not lineage II idInlB. Taken together, obtained results suggested that phylogenetically defined substitutions in idInlB provide functional distinctions and might be involved in phylogenetically determined differences in virulence potential.

The facultative intracellular pathogen Listeria monocytogenes causes a severe food-borne infection in humans and animals. L. monocytogenes invasion factor InlB interacts with the tyrosine kinase c-Met via the N-terminal internalin domain. Previously, distinct variants of the InlB internalin domain (idInlB) have been described in L. monocytogenes field isolates. Three variants were used to restore full-length InlB expression in the L. monocytogenes strain EGDeΔinlB. Obtained isogenic L. monocytogenes strains were tested in the invasion assay and intravenous, intraperitoneal, and intragastric models of infection in mice. All idInlBs were functional, restored InlB activity as an invasion factor, and improved invasion of the parental strain EGDeΔinlB into human kidney HEK23 cells. Meanwhile, distinct idInlBs provided different mortality rates and bacterial loads in internal organs. When recombinant strains were compared, the variant designated idInlB14 decreased severity of disease caused by intravenous and intraperitoneal bacterial administration, whereas this variant improved intestine colonization and stimulated intragastric infection. Obtained results demonstrated that naturally occurring idInlBs differed in their impact on severity of L. monocytogenes infection in mice in dependence on the infection route.

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We used multivirulence locus sequence typing to analyze 68 Yersinia pseudotuberculosis isolated in Russia during 1973–2014, including 41 isolates from patients with Far East scarlet-like fever. Four genotypes were found responsible, with 1 being especially prevalent. Evolutionary analysis suggests that epidemiologic advantages could cause this genotype’s dominance.