THE GENUS - SALMONELLA
Salmonella organisms are facultative anaerobic gram-negative rods within the family of Enterobacteriaceae (Yan et al., 2003). Classically, the members of this genus are motile by peritrichous flagella except Salmonella Pullorum and Salmonella gallinarum, which lack flagella (Narapati, 2007).
Salmonella grow optimally at 35oC to 37oC, catabolize a variety of carbohydrates into acid and gas, use citrate as the sole carbon source, produces H2S and decarboxylate lysine and ornithine to cadaverine and putrescine respectively (Narapati, 2007).
Historically Salmonella catabolized glucose and lysine, but failed to metabolize lactose, sucrose and urea, however due to the widespread exchange of genetic elements between compatible bacterial strains in the environment, atypical Salmonella biotypes that cannot decarboxylate lysine or that readily use lactose, sucrose and urea, have been isolated (Narapati, 2007).
Many serotypes in the group are closely related to each other by somatic and flagellar antigens and most strains show diphasic variation of the flagellar antigens.
The genus Salmonella comprises two species:
- Salmonella enteric (which is divided into six subspecies: Salmonella enterica subspecies enterica (I), Salmonella enterica subspecies salamae (II), Salmonella enterica subspecies arizonae (IIIa), Salmonella enterica subspecies diarizonae (IIIb), Salmonella enterica subspecies houtenae (IV) and Salmonella enterica subspecies indica (VI))
- Salmonella bongori (formerly called Salmonella enterica subspecies bongori).
Species and subspecies can be distinguished on the basis of differential characters and through antigenic formulae. A recent report from the Centre for Infectious Disease Research and Policy classifies members of the Salmonella species into more than 2541 serotypes (serovars) according to their somatic (O) and flagellar (H) antigens (Narapati, 2007).
Several schemes based on biochemical characteristics, DNA homology, and enzyme electrophoretic patterns have been used for the taxonomic classification of Salmonella. Salmonella nomenclature is complex, and scientists use different systems to refer to and communicate about this GENUS - SALMONELLA.
The current usage often combines several nomenclatural systems that inconsistently divide the genus into species, subspecies, subgenera, groups, subgroups, and serotypes (serovars) which causes confusion (Brenner et al., 2000). The genus Salmonella has two systems of nomenclature in circulation.
One system proposed by Le Minor and Popoff in the 1980s, which has received a wide acceptance although it does not conform to the rules of bacteriological code and the other which conforms to the bacteriological code, but used by the minority. The present problem is that two systems of nomenclature are in the use for the members of the genus Salmonella (Narapati, 2007).
The nomenclature of the genus Salmonella has evolved from the initial one serotype-one species concept proposed by Kauffmann on the basis of the serologic identification of O (somatic) and H (flagellar) antigens. A capsular polysaccharide, the Vi antigen is present on Salmonella Typhi and few other serovars of Salmonella, including Salmonella Dublin (Narapati, 2007).
The current nomenclature used by Center for Disease Control and Prevention is based on the recommendations from the WHO collaborating centre and it adequately addresses the concern and requirements of clinical and public health microbiologists (Deb and Kapoor, 2005). According to the CDCP system, the genus Salmonella contains two species, each of which contains multiple serotypes (Brenner et al., 2000).