Salmonella and Salmonellosis (page 2)
(This chapter has 5 pages)
© Kenneth Todar, PhD
As with all Enterobacteriaceae, the genus Salmonella
three kinds of major antigens with diagnostic or identifying
somatic, surface, and flagellar.
Somatic (O) or Cell Wall Antigens
Somatic antigens are heat stable and alcohol resistant.
studies individualize a large number of antigenic factors, 67 of which
are used for serological identification. O factors labeled with the
number are closely related, although not always antigenically
Surface (Envelope) Antigens
Surface antigens, commonly observed in other genera of enteric bacteria
(e.g., Escherichia coli and Klebsiella), may be found
some Salmonella serovars. Surface antigens in Salmonella
may mask O antigens, and the bacteria will not be agglutinated with O
One specific surface antigen is well known: the Vi antigen. The Vi
occurs in only three Salmonella serovars (out of about 2,200):
Paratyphi C, and Dublin. Strains of these three serovars may or may not
have the Vi antigen.
Flagellar (H) Antigens
Flagellar antigens are heat-labile proteins. Mixing salmonella cells
with flagella-specific antisera gives a characteristic pattern of
agglutination (bacteria are loosely attached to each other by their
and can be dissociated by shaking). Also, antiflagellar antibodies can
immobilize bacteria with corresponding H antigens.
A few Salmonella entericaserovars (e.g.,
Typhi) produce flagella which always have the same antigenic
Such an H antigen is then called monophasic. Most Salmonella
however, can alternatively produce flagella with two different H
specificities. The H antigen is then called diphasic. For example,
cells can produce flagella with either antigen i or antigen 1,2. If a
is derived from a bacterial cell with H antigen i, it will consist of
with i flagellar antigen. However, at a frequency of 10-3- 10-5,
bacterial cells with 1,2 flagellar antigen pattern will appear in this
Figure 2. Flagellar stain of
a Salmonella Typhi. Like E. coli, Salmonella are motile
means of peritrichous flagella. A close relative that causes enteric
is the bacterium Shigella. Shigella is not motile, and
it can be differentiated from Salmonella on the bais of a
test or a flagellar stain. (CDC)
The principal habitat of the salmonellae is the intestinal tract of
humans and animals. Salmonella serovars can be found
in one particular host, can be ubiquitous, or can have an unknown
Typhi and Paratyphi A are strictly human serovars that may
grave diseases often associated with invasion of the bloodstream.
in these cases is transmitted through fecal contamination of water or
Gallinarum, Abortusovis, and Typhisuis are, respectively, avian, ovine,
and porcine Salmonella serovars. Such host-adapted serovars
grow on minimal medium without growth factors (contrary to the
Ubiquitous (non-host-adapted) Salmonella serovars (e.g.,
cause very diverse clinical symptoms, from asymptomatic infection to
typhoid-like syndromes in infants or certain highly susceptible animals
(mice). In human adults, ubiquitous Salmonella organisms are
responsible for foodborne toxic infections.
The pathogenic role of a number of Salmonella serovars is
This is especially the case with serovars from subspecies II to
A number of these serovars have been isolated rarely (some only once)
a systematic search in cold-blooded animals.
Salmonella in the Natural Environment
Salmonellae are disseminated in the natural environment (water, soil,
sometimes plants used as food) through human or animal excretion.
and animals (either wild or domesticated) can excrete Salmonella
either when clinically diseased or after having had salmonellosis, if
remain carriers. Salmonella organisms do not seem to multiply
in the natural environment (out of digestive tracts), but they can
several weeks in water and several years in soil if conditions of
humidity, and pH are favorable.