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More than any other genus, Salmonella has been a favorite of those who love to subdivide and apply names to biologic systems. At one time, there were over 2000 names for vari-ous members of this genus, often reflecting colorful aspects of place or circumstances of the original isolation (eg, S. budapest, S. seminole, S. tamale, S. oysterbeds). This has now been reduced to a single species, Salmonella enterica, with the previous species names relegated to the status of serotypes. All of this is made particularly robust by the fact that, in addition to a large number of the LPS O and some capsular K antigens, the flagellar H antigens of most Salmonella undergo phase variation. This adds the prospect of two sets of H antigens to the already complex system. As in Shigella, the specific O antigens are organized into serogroups (eg, A, B, . . . K, and so on) to which the two H and K (if present) antigen designations are appended to achieve the full antigenic formula. It is not difficult to understand why microbiologists, when confronted with a salmonella with the antigenic formula O:group B [1,4,12] H:I;1,2, still prefer to call itSalmonella typhimurium. The proper name for this organism is Salmonella enterica serotype Typhimurium, but indulging in the convenience of elevating the serotype to species status is still common.
Another feature distinguishing Salmonella serotypes is their host range. Some are highly adapted to particular mammals or amphibians, and others infect a broad range of hosts. Of interest for medical microbiology are those that infect humans and other ani-mals and those strictly adapted to humans and higher primates. S. enterica serotype Ty-phimurium is the prototype for the former and S. enterica serotype Typhi for the latter. In the following discussions, Typhi will be used for the strictly human species that produce enteric (typhoid) fever. Unless otherwise specified, S. enterica will be used for the serotypes that are able to infect animals or humans and typically cause gastroenteritis in the latter.
Salmonellae possess multiple types of pili, one of which is morphologically and functionally similar to the E. coli type 1 pili, which bind D-mannose receptors on various eukaryotic cell types. Most strains are motile through the action of their flagella.
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