Listeria are Gram-positive, motile, facultative anaerobic bacteria; L.welshimeri (serovar 6b) is one of 4 nonpathogenic members of the genus. There are 2 pathogenic members, L.monocytogenes (also fully sequenced) and L.ivanovii. L.welshimeri was first isolated from decaying plants. Biochemical properties such as presence of catalase activity, absence of oxidase activity and acid production from fermentation of D-xylose or alpha-methyl-D-mannoside but not from L-rhamnose or D-mannitol are used to distinguish it from other Listeria. Comparisons between the 4 currently fully-sequenced genomes L.welshimeri, L.innocua (another non-pathogenic Listeria) and L.monocytogenes indicate that the non-pathogenic Listeria probably evolved from a virulent ancestor by loss of the virulence gene cluster region. Genes lost include those coding for virulence, general intracellular survival genes, surface-associated LPXTG- and LRR-containing proteins and proteins involved in survival at elevated temperatures. L.welshimeri has undergone further gene loss (233 genes compared to L.innocua and L.monocytogenes) and acquired other novel genes (311 genes compared to L.innocua and L.monocytogenes), probably as a result of horizontal gene transfer. Genes exclusively present in L.welshimeri include those for uptake and utilization systems for plant energy sources, indicating an adaptation to a plant saprophytic lifestyle. It contains a single putative prophage inserted in a region that in the related pathogen L.ivanovii has pathogenicity island.