Listeria monocytogenes is an important foodborne pathogen
that comprises four genetic lineages: I, II, III, and IV. Of
these, lineage II is frequently recovered from foods and
environments and responsible for the increasing incidence of
human listeriosis. In this study, the phylogenetic structure
of lineage II was determined through sequencing analysis
of the ascB-dapE internalin cluster. Fifteen sequence types
proposed by multilocus sequence typing based on nine
housekeeping genes were grouped into three distinct
sublineages, IIA, IIB, and IIC. Organization of the ascBdapE
internalin cluster could serve as a molecular marker
for these sublineages, with inlGHE, inlGC2DE, and inlC2DE
for IIA, IIB, and IIC, respectively. These sublineages
displayed specific genetic and phenotypic characteristics.
IIA and IIC showed a higher frequency of recombination
(ρ/θ). However, recombination events had greater effect
(r/m) on IIB, leading to its high nucleotide diversity.
Moreover, IIA and IIB harbored a wider range of
internalin and stress-response genes, and possessed higher
nisin tolerance, whereas IIC contained the largest portion
of low-virulent strains owing to premature stop codons in
inlA. The results of this study indicate that IIA, IIB, and
IIC might occupy different ecological niches, and IIB might
have a better adaptation to a broad range of environmental
niches.

Keywords: L. monocytogenes, lineage II, sublineage, ascB-dapE internalin cluster