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Tag words: bacteria, enteric bacteria, microbiology, microbe, Salmonella, Salmonella Enterica, Salmonella Typhi, S. Typhimurium, S. Enterica, typhoid fever, enteric fever, salmonellosis, food poisoning, gastroenteritis.


Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Enterobacteriales
Family: Enterobacteriaceae
Genus: Salmonella
Species: e.g. S. enterica

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Salmonella and Salmonellosis (page 1)

(This chapter has 5 pages)

© Kenneth Todar, PhD

Salmonella  is a Gram-negative facultative rod-shaped bacterium in the same proteobacterial family as Escherichia coli, the family Enterobacteriaceae, trivially known as "enteric" bacteria. Salmonella is nearly as well-studied as E. coli from a structural, biochemical and molecular point of view, and as poorly understood as E. coli from an ecological point of view. Salmonellae live in the intestinal tracts of warm and cold blooded animals. Some species are ubiquitous. Other species are specifically adapted to a particular host. In humans, Salmonella are the cause of two diseases called salmonellosis: enteric fever (typhoid), resulting from bacterial invasion of the bloodstream, and acute gastroenteritis, resulting from a foodborne infection/intoxication.

Discovery of the Typhoid Bacillus

At the beginning of the 19th century, typhoid was defined on the basis of clinical signs and symptoms and pathological (anatomical) changes. However, at this time, all sorts of enteric fevers were characterized as "typhoid".

In 1880s, the typhoid bacillus was first observed by Eberth in spleen sections and mesenteric lymph nodes from a patient who died from typhoid. Robert Koch confirmed a related finding by Gaffky and succeeded in cultivating the bacterium in 1881. But due to the lack of differential characters, separation of the typhoid bacillus from other enteric bacteria was uncertain.

In 1896, it was demonstrated that the serum from an animal immunized with the typhoid bacillus agglutinated (clumped) the typhoid bacterial cells, and it was shown that the serum of patients afflicted with typhoid likewise agglutinated the typhoid bacillus. Serodiagnosis of typhoid was thus made possible by 1896.

Figure 1. Salmonella typhi, the agent of typhoid. Gram stain. (CDC)

Salmonella Nomenclature

The genus Salmonella is a member of the family Enterobacteriaceae, It is composed of bacteria related to each other both phenotypically and genotypically. Salmonella DNA base composition is 50-52 mol% G+C, similar to that of Escherichia, Shigella, and Citrobacter. The bacteria of the genus Salmonella are also related to each other by DNA sequence. The genera with DNA most closely related to Salmonella are Escherichia, Shigella, and Citrobacter. Similar relationships were found by numerical taxonomy and 16S ssRNA analysis.

Salmonella nomenclature has been controversial since the original taxonomy of the genus was not based on DNA relatedness, rather  names were given according to clinical considerations, e.g., Salmonella typhi, Salmonella cholerae-suis, Salmonella abortus-ovis, and so on. When serological analysis was adopted into the Kauffmann-White scheme in 1946, a Salmonella species was defined  as "a group of related fermentation phage-type" with the result that each Salmonella serovar was considered as a species. Since the host-specificity suggested by some of these earlier names does not exist (e.g., S. typhi-murium, S. cholerae-suis are in fact ubiquitous), names derived from the geographical origin of the first isolated strain of the newly discovered serovars were next chosen, e.g., S. london, S. panama, S. stanleyville.

Susequently it was found that all Salmonella serovars form a single DNA hybridization group, i.e., a single species composed of seven subspecies, and thenomenclature had to be adapted. To avoid confusion with the familiar names of serovars, the species name Salmonella enterica was proposed with the following names for the subspecies:
enterica  I
salamae II
arizonae IIIa
diarizonae IIIb
houtenae IV
bongori V
indica VI
Each subspecies contains various serovars defined by a characteristic antigenic formula.

Since this formal Latin nomenclature may not be clearly understood by physicians and epidemiologists, who are the most familiar with the names given to the most common serovars, the common serovars names are kept for subspecies I strains, which represent more than 99.5% of the Salmonella strains isolated from humans and other warm-blooded animals. The vernacular terminology seems preferred in medical practice, e.g., Salmonella ser. Typhimurium (not italicized) or shorter Salmonella (or S.) Typhimurium.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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