ABSTRACT
Classi©cation. The genus Listeria is classiŠed taxonomically in the family Listeriaceae, order Bacillales, class Bacilli, phylum Furmicutes. The only other genus in the family Listeriaceae is Brochothrix (consisting of two species, Brochothrix campestris and Brochothrix thermosphacta). Until recently, six closely related, gram-positive bacterial species have been recognized in the genus Listeria-that is, L. monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. welshimeri, and L. grayi, with L. ivanovii being further separated into two subspecies, L. ivanovii subsp. londoniensis and L. ivanovii subsp. ivanovii.1,2 Whereas L. monocytogenes is a facultative intracellular pathogen for humans and animals, L. ivanovii (formerly L. monocytogenes serotype 5) mainly infects ungulated animals (e.g., sheep and cattle), and the other four species are essentially saprophytes that have adapted for free living in soil and decaying vegetation.1,3 In late 2009, two additional Listeria species were described. Having originated from soil and water samples at the Finger Lakes National Forest in New York, L. marthii sp. nov. demonstrates a close phylogenetic relatedness to L. monocytogenes and L. innocua, and a more distant relatedness to
L. welshimeri, L. seeligeri, L. ivanovii, and L. grayi. Given its absence of a homologue of the L. monocytogenes virulence gene island, L. marthii is likely nonpathogenic.4 L. rocourtiae sp. nov. was identiŠed from precut lettuce in Austria. This species is distinguishable from other Listeria species by using phenotypic tests, and its type strain is avirulent as assessed by in vitro cell culture and in vivo mouse model.5
