Salmonellae: Taxonomy, Genomics and Antimicrobial Resistance
from: Foodborne and Waterborne Bacterial Pathogens: Epidemiology, Evolution and Molecular Biology (Edited by: Shah M. Faruque). Caister Academic Press, U.K. (2012)
The genus Salmonella has three species namely Salmonella enterica, Salmonella bongori and Salmonella subterranean. The type species S. enterica is further classified into six subspecies: enterica (subsp. I), arizonae (subsp. IIIa), diarizonae (subsp. IIIb), houtenae (subsp. IV), indica (subsp. VI), and salamae (subsp. II). Salmonella strains belong to over 50 serogroups based on the O antigen, and to over 2500 serovars (each having a unique combination of somatic O, flagellar H1 and H2 antigens). Most of these serovars (1,531) belong to Salmonella subsp., enterica, and cause more than 99% of the diseases in humans including gastroenteritis and enteric fever (typhoid). Genome sequencing and comparative genomic analysis of 28 S. enterica serovars identified similarity of core regions of the genomes, together with evidence of recombination and rearrangement, genomic degradation, pseudogenes and clonal diversity both within and among the serovars. Genomic comparisons of host-restricted (S. Typhi, S. Paratyphi and S. Gallinarum) and host-adapted (S. Typhimurium and S. Enteridis) S. enterica serovars indicate that genomic degradation is a common evolutionary mechanism for host adaptation and increased pathogenicity of Salmonella. Drug resistances in Salmonella is mainly due to Salmonella genomic island 1 (an integrative mobile element) carrying various antibiotic resistance gene clusters, and to conjugative R plasmids which confer resistance to many antibiotics including extended-spectrum cephalosporins. Continuous genetic re-assortment in Salmonella leading to increased virulence and the emergence of resistance to multiple drugs are of significant public health concern read more ...