Listeria

Abstract

The genus Listeria is a member of the family Listeriaceae within the order Bacillales and currently comprises ten distinct species. Listeria spp. are gram-positive, nonsporulating, facultatively anaerobic bacteria of low G+C content that are present ubiquitously in nature and have been isolated from diverse environmental sources. The genus Listeria harbors two pathogenic species, Listeria monocytogenes and L. ivanovii, and eight nonpathogenic species that include L. innocua, L. seeligeri, L. welshimeri, L. grayi, L. marthii, L. rocourtiae, L. fleischmannii, and L. weihenstephanensis. Based on their 16S rRNA sequences, L. fleischmannii and L. grayi branch deeply and are clearly separated from the remaining species. Even though classified as mesophilic bacteria, Listeria spp. are characterized by their ability to grow at low temperatures and have high tolerance ranges to both salt concentrations (10%) as well as a broad range of pH. Listeriosis, the infectious disease caused by L. monocytogenes, is an extremely serious, life-threatening foodborne disease with high mortality rate documented by several outbreaks. Contamination with L. monocytogenes is critical for the food industry and is among the leading microbiological causes of food recalls. The pathogenic bacterium L. monocytogenes is morphologically indistinguishable from other Listeria species and often causes nonspecific clinical symptoms; therefore, laboratory testing is required to differentiate L. monocytogenes from other Listeria species and to diagnose listeriosis. Comparative analysis of full genome sequences reveals an open pan-genome and that Listeria genome evolution is dictated by limited gene acquisition and gene loss. Based on these analyses, the diversity of modern listerial species evolved through loss of virulence properties rather than virulence gene acquisition.