Listeria monocytogenes is an opportunistic intracellular pathogen that has become an important cause of human foodborne infections worldwide. Given its close relationship to other Listeria species and its tendency to produce non-specific clinical symptoms, the availability of rapid, sensitive and specific diagnostic tests for the differentiation of L. monocytogenes from other Listeria species is helpful for selecting appropriate treatment regimens. In addition, with L. monocytogenes comprising a diversity of strains of varying pathogenicity, the ability to precisely track the strains involved in listeriosis outbreaks and speedily determine their pathogenic potential is critical for the control and prevention of further occurrences of this deadly disease. Extensive research in recent decades has revealed significant insights regarding the molecular mechanisms of L. monocytogenes infection. This in turn has facilitated the development of laboratory procedures for enhanced detection and identification of L. monocytogenes, and has also contributed to the implementation of improved control and prevention strategies against listeriosis. The purpose of this review is to summarize recent progress in the species-specific identification, subtyping and virulence determination of L. monocytogenes strains, and to discuss future research needs pertaining to these important areas of listeriosis.

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Listeria monocytogenes is an opportunistic bacterial pathogen that is an important cause of human food-borne illness worldwide. However, L. monocytogenes strains demonstrate considerable variation in pathogenic potential. In this report, virulent and avirulent L. monocytogenes isolates were compared by using a comparative screening strategy. Two clones were identified that contained DNA that was only present in virulent L. monocytogenes strains. PCR primers were designed for three genes from these clones and for five other selected L. monocytogenes genes. All eight primer sets predominantly detected virulent L. monocytogenes isolates, as determined by a mouse virulence assay; one of the putative internalin genes, lmo2821, was detected in all strains that were considered to be virulent. Primers from these eight genes were then tested by PCR against a larger panel of bacterial strains; each of the genes was detected predominantly in clinical or food L. monocytogenes isolates, rather than environmental isolates. The findings from this study suggest that virulent L. monocytogenes strains may possess genes that are not present in avirulent isolates, which could serve as markers for PCR assessment of L. monocytogenes virulence.

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...

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