Antigenic Structure

As with all Enterobacteriaceae, the genus Salmonella has three kinds of major antigens with diagnostic or identifying applications: somatic, surface, and flagellar.

Somatic (O) or Cell Wall Antigens
Somatic antigens are heat stable and alcohol resistant. Cross-absorption studies individualize a large number of antigenic factors, 67 of which are used for serological identification. O factors labeled with the same number are closely related, although not always antigenically identical.

Surface (Envelope) Antigens
Surface antigens, commonly observed in other genera of enteric bacteria (e.g., Escherichia coli and Klebsiella), may be found in some Salmonella serovars. Surface antigens in Salmonella may mask O antigens, and the bacteria will not be agglutinated with O antisera. One specific surface antigen is well known: the Vi antigen. The Vi antigen occurs in only three Salmonella serovars (out of about 2,200): Typhi, 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 flagella and can be dissociated by shaking). Also, antiflagellar antibodies can immobilize bacteria with corresponding H antigens.

A few Salmonella entericaserovars (e.g., Enteritidis, Typhi) produce flagella which always have the same antigenic specificity.  Such an H antigen is then called monophasic. Most Salmonella serovars, however, can alternatively produce flagella with two different H antigenic specificities. The H antigen is then called diphasic. For example, Typhimurium cells can produce flagella with either antigen i or antigen 1,2. If a clone is derived from a bacterial cell with H antigen i, it will consist of bacteria 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 clone.

Figure 2. Flagellar stain of a Salmonella Typhi. Like E. coli, Salmonella are motile by means of peritrichous flagella. A close relative that causes enteric infections is the bacterium Shigella. Shigella is not motile, and therefore it can be differentiated from Salmonella on the bais of a motility test or a flagellar stain. (CDC)

Habitats

The principal habitat of the salmonellae is the intestinal tract of humans and animals. Salmonella serovars can be found predominantly in one particular host, can be ubiquitous, or can have an unknown habitat. Typhi and Paratyphi A  are strictly human serovars that may cause  grave diseases often associated with invasion of the bloodstream. Salmonellosis in these cases is transmitted through fecal contamination of water or food.  Gallinarum, Abortusovis, and Typhisuis are, respectively, avian, ovine, and porcine Salmonella serovars. Such host-adapted serovars cannot grow on minimal medium without growth factors (contrary to the ubiquitous Salmonella serovars).

Ubiquitous (non-host-adapted) Salmonella serovars (e.g., Typhimurium) cause very diverse clinical symptoms, from asymptomatic infection to serious typhoid-like syndromes in infants or certain highly susceptible animals (mice). In human adults, ubiquitous Salmonella organisms are mostly responsible for foodborne toxic infections.

The pathogenic role of a number of Salmonella serovars is unknown. This is especially the case with serovars from subspecies II to VI.  A number of these serovars have been isolated rarely (some only once) during 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. Humans and animals (either wild or domesticated) can excrete Salmonella either when clinically diseased or after having had salmonellosis, if they remain carriers. Salmonella organisms do not seem to multiply significantly in the natural environment (out of digestive tracts), but they can survive several weeks in water and several years in soil if conditions of temperature, humidity, and pH are favorable.