ABSTRACT
The genus Listeria comprises a group of Gram-positive, rod-shaped bacteria, which are classi-ed taxonomically into six species (i.e., L. monocytogenes, L. ivanovii, L. innocua, L. seeligeri, L. welshimeri, and L. grayi). Whereas L. monocytogenes is a facultative intracellular pathogen of both humans and animals, L. ivanovii (previously known as L. monocytogenes serotype 5) mainly infects ruminant animals (e.g., sheep and cattle). The other four species are free-living saprophytes with no apparent pathogenic inclination. Based on serological reactions, Listeria species are divided into over 15 distinct serotypes, with L. monocytogenes consisting of serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7; L. ivanovii of serotype 5, and other Listeria species of serotypes 6a and 6b as well as several shared serotypes with L. monocytogenes.1,2 Using genetic typing methods, L. monocytogenes is separated into three lineages: lineage I is composed of serotypes of 1/2b, 3b, 4b, 4d, and 4e; lineage II of serotypes 1/2a, 1/2c, 3a, and 3c; and lineage III of serotypes 4a and 4c.3,4 In addition, lineage III can be subdivided into subgroups IIIA, IIIB, and IIIC.5,6
Listeria spp. demonstrate considerable morphological, biochemical, and molecular resemblances and occupy similar ecological niches in the environment. Given their renowned ability to withstand arduous external conditions such as wide pH, temperature, and salt ranges,7,8 there is no surprise that Listeria spp. are distributed in a diverse range of environments and have been isolated from soil, water, efuents, foods, wildlife, and domestic animals as well as humans. Since Listeria spp. are co-present in various environmental, food, and clinical specimens, they often add complexity to the laboratory identication of L. monocytogenes and diagnosis of listeriosis. Indeed, it has only become feasible in recent decades to clarify the taxonomical status of Listeria spp and to discriminate L. monocytogenes from other Listeria spp in a reliable and precise manner with the application of molecular techniques. Therefore, any discussion on L. monocytogenes epidemiology will be incomplete and impossible without mentioning the other nonmonocytogenes Listeria spp.
