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CLASSIFICATION

There has been some controversy over the speciation of this genus; currently the species name is S. enterica, which is used for types previously considered as subspecies (subgenus) I. The other species are S. salamae (subspecies II), arizonae (subspecies IIIa), diarizonae (subspecies IIIb), houtenae (subspecies IV), bongori (subspecies V) and indica (subspecies VI). These subspecies are further subdivided into serovars (serological variants; see Serology).

BIOCHEMISTRY

They do not produce indole, ferment glucose by the mixed acid fermentation, but do not produce phenylalaninedeaminase or urease, utilize citrate as sole carbon source, produce H2S (except some strains of serovar Typhi) Most are motile, ferment a variety of carbohydrates and decarboxylate arginine, lysine and/or ornithine.

Many genetic studies have been done on Salmonella species, particularly on serovar Typhimurium. Its chromosome is very similar to that of Escherichia coli and consists of a single circular DNA molecule consisting of about 4 x 10⁶ base pairs with a molecular weight of 4 x 10⁹ and a total length of about 1.4mm. Many of the genes have been mapped.

Salmonella can be subdivided according to their somatic (cell-wall) or O antigens and their flagellar or H antigens. Currently there are recognized over 50 distinct O types or partial O antigens, leading to strains being designated as carrying a number of partial antigens as O:4,5,12 or just one as O:35. There are also over 50 recognized H antigens which may again be subtyped as partial antigens. In addition the phenomenon of phase-variation occurs among many Salmonella types, thus they may switch from making flagellae with one type of specificity to another. Some triphasic serovars are also known. H antigens are designated with a single digit numeral, a lower case letter of the alphabet or with the letter "z" with a suffix. Currently over 2000 possible OH serovars are known. Examples of serovar formulae are:

Paratyphi A: 1,2,12:a:-Typhimurium: 1,4,[5],12:i:1,2
Anatum:      3,10:e,h:1,6Minnesota:   21:b:e,n,x
Ealing:      35:g,m,s:-Waycross:    41:z4,z23:-

There are also capsular and fimbrial antigens.

Enterotoxins:
Salmonella strains may produce a Heat Labile enterotoxin related to the E. coli Heat Labile enterotoxin (LT) or Cholera Toxin (CT).

Verotoxins or Shiga-like toxins:
The two terms are interchangeable. Cytotoxins related to but distinct from those produced by E. coli or Shigella may also be produced.

Other Toxins:
Currently there is no information on other toxins..

Haemolysins:
Salmonella strains are not haemolytic.

Adhesins & Fimbriae:
The Vi antigen produced by the three serovars Typhi, Paratyphi C and Dublin may be associated with virulence.
Fimbrae can be produced by Salmonella strains.

NORMAL FLORA

They may be part of the commensal flora of some cold-blooded animals. They are generally causes of intestinal infections in humans and warm-blooded animals, but may be carried without causing symptoms.

Enteric Infections:
They cause gastroenteritis and some types can cause enteric fever (typhoid).
Extraintestinal Infections:
These are rarer and usually are complications of intestinal infections.
Animal Infections:
They cause similar infections in animals as some of the human ones, although animals may also be carriers.

Salmonella can be grown very easily on most microbiological media. Generally the production of non-lactose-fermenting colonies on media containing bile-salts is a strong indication of the presence of Salmonella or Shigella . The ability of most strains to produce H2S is also very useful. Media to selectively enrich for Salmonellae have been developed.

Salmonella can certainly survive but may not actively grow in many environmental waters. As Salmonella are present in the faeces of humans and most animals they are often present in faecally polluted waters.

There are no specific industrial uses except for vaccine production. However, as the Salmonella genome is so similar to the E. coli genome, they may be used in future.

Vaccines against typhoid fever have been in use for many years.
Parenteral vaccines use killed bacteria but are not as effective as oral vaccines using either killed or living avirulent variants.

Enterobacteriaceae

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