SALMONELLA
BACTERIOLOGY
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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