Listeria monocytogenes Subgroups IIIA, IIIB, and IIIC Delineate Genetically Distinct Populations with Varied Pathogenic Potential

Abstract

Listeria monocytogenes lineage III strains belonging to subgroups IIIA (n ‫؍‬ 8), IIIB (n ‫؍‬ 5), and IIIC (n ‫؍‬ 6) were examined along with other known serotype strains (n ‫؍‬ 11) by PCR and Southern hybridization using several recently described species-, virulence-, and serotype-specific primers and probes. The virulence of seven representative lineage III strains was then evaluated in mice via the intraperitoneal route. The results suggest that subgroup IIIA consists of typical rhamnose-positive avirulent serotype 4a and virulent serotype 4c strains, subgroup IIIC consists of atypical rhamnose-negative virulent serotype 4c strains, and subgroup IIIB consists of atypical rhamnose-negative virulent non-serotype 4a and non-serotype 4c strains, some of which may be related to serotype 7. It is possible that subgroup IIIB (including serotype 7) may represent a novel subspecies within L. monocytogenes.Listeria monocytogenes is a facultative intracellular bacterium that displays considerable variations in virulence and pathogenicity (5,6,8,19). On the basis of the serological reactions between Listeria somatic (O)/flagellar (H) antigens and their corresponding antisera, L. monocytogenes is divided into at least 12 serotypes (i.e., 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7) (17). By using various genetic typing techniques, L. monocytogenes can be further classified into three lineages, of which lineage I encompasses serotypes 1/2b, 3b, 4b, 4d, and 4e; lineage II includes serotypes 1/2a, 1/2c, 3a, and 3c; and lineage III comprises serotypes 4a and 4c (2,10,13,20). However, the lineage status of serotype 7 remains unclear due to limited availability of such strains. The clinical relevance of the serological and genetic typing schemes is demonstrated by the observations that serotypes 1/2a, 1/2c, 1/2b, and 4b (of lineages I and II) are involved in over 98% of the documented human listeriosis cases and that serotypes 4a and 4c (of lineage III) are rarely associated with outbreaks of the disease despite their frequent isolation from a variety of food and environmental specimens (20).Although a general consensus on the compositions and divisions of lineages I and II exists, some uncertainty remains concerning the phenotypic characteristics and taxonomic status of lineage III. Within lineage III, three genetically distinct subgroups (i.e., IIIA, IIIB, and IIIC) have been identified after a recent comparative analysis of actA and sigB gene sequences from 46 strains (14). Phenotypically, lineage IIIA strains behave like typical L. monocytogenes in their ability to ferment rhamnose, whereas lineage IIIB and IIIC strains are notably deficient in rhamnose utilization (14). Given that positive rhamnose activity has been incorporated in various rapid identification kits (e.g., API Listeria [BioMerieux] and Micro-ID [Organon Technika]) for differentiation of L. monocytogenes from other nonpathogenic Listeria species (16), the identification of several unusual, rhamnose-negative lineage III strains (e.g., FSL-J1-158...

Multilocus Genotyping Assays for Single Nucleotide Polymorphism-Based Subtyping of Listeria monocytogenes Isolates

“…Phylogenetic analysis of VGC, LMO, and INL sequences confirmed previous reports (22,33,38) that lineage IV isolates are not part of a monophyletic lineage III and indicated that they represent an early divergence within L. monocytogenes (Fig. 2).…”

“…Toward that end, we recently described a singlewell DNA sequence-based subtyping method utilizing multilocus genotyping (MLGT) of single-nucleotide polymorphisms (SNPs) segregating within L. monocytogenes lineage I that provides high discriminatory power and good epidemiological concordance (5). While we previously focused on subtyping lineage I isolates because of their association with epidemic listeriosis, four major evolutionary lineages have been described for L. monocytogenes (22,33,36,38,46,47). L. monocytogenes isolates from lineage II are common contaminants of RTE and other food products (13,46), contribute significantly to sporadic listeriosis in humans (18), and include a previously described epidemic…”

Antimicrobial susceptibility and serotype distribution of Listeria monocytogenes isolates obtained from raw milk cheese samples sold in Nigde

“…Lineage II isolates ( 1/2a, 1/2c and 3a serotypes) are more frequently isolated from foods and food environments compared to lineage I isolates (12,18). One of the isolates obtained from cheese samples in this study belonged to serotype 1/2a which is in agreement with previous studies in several countries (10,17).…”

“…Ninety six percent of the human cases of listeriosis are reported to be caused only by three serotypes among the 12 serotypes that are 1/2a, 1/2b and 4b. The other serotypes (especially 4a) are reported to be found mostly in foods and animals but rarely responsible for human L. monocytogenes infections (12).…”

Genome Sequence of the Nonpathogenic Listeria monocytogenes Serovar 4a Strain M7

“…Listeria monocytogenes encompasses a diversity of strains with various virulence and pathogenicity levels (10,19). Serovars 4b, 1/2a, and 1/2b account for the majority of human listeriosis cases, whereas other serovars, particularly serovar 4a, are rarely associated with listeriosis and are seldom isolated from foods (3,18).…”