Salmonella
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Salmonellosis Microchapters |
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Diagnosis |
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Case Studies |
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Salmonella On the Web |
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American Roentgen Ray Society Images of Salmonella |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2] Jolanta Marszalek, M.D. [3]
Overview
Salmonella is a genus of rod-shaped Gram-negative enterobacteria that causes typhoid fever, paratyphoid fever, and foodborne illness.[1] Salmonella species are motile and produce hydrogen sulfide.[2]
Taxonomy
Cellular organism; Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae[3]
Biology
Infectious Cycle
Salmonella is a Gram-negative bacterium. In a clinical laboratory, it is usually isolated on MacConkey agar, XLD agar, XLT agar, DCA agar, or Önöz agar. Because they cause intestinal infections and are greatly outnumbered by the bacteria normally found in the healthy bowel, primary isolation requires the use of a selective medium, so use of a relatively non-selective medium such as CLED agar is not often practiced. Numbers of salmonella may be so low in clinical samples that stools are routinely also subjected to "enrichment culture", where a small volume of stool is incubated in a selective broth medium, such as selenite broth or Rappaport Vassiliadis soya peptone broth, overnight. These media are inhibitory to the growth of the microbes normally found in the healthy human bowel, while allowing salmonellae to become enriched in numbers. Salmonellae may then be recovered by inoculating the enrichment broth on one or more of the primary selective media. On blood agar, they form moist colonies about 2 to 3 mm in diameter. When the cells are grown for a prolonged time at a range of 25—28°C, some strains produce a biofilm, which is a matrix of complex carbohydrates, cellulose and proteins. The ability to produce biofilm (a.k.a. "rugose", "lacy", or "wrinkled") can be an indicator of dimorphism, which is the ability of a single genome to produce multiple phenotypes in response to environmental conditions. Salmonellae usually do not ferment lactose; most of them produce hydrogen sulfide which, in media containing ferric ammonium citrate, reacts to form a black spot in the centre of the creamy colonies.
Structure
Classification
Salmonella taxonomy is complicated.[4][5] As of December 7, 2005, there are two species within the genus: S. bongori (previously subspecies V) and S. enterica (formerly called S. choleraesuis), which is divided into six subspecies:
- I—enterica
- II—salamae
- IIIa—arizonae
- IIIb—diarizonae
- IV—houtenae
- V—obsolete (now designated S. bongori)
- VI—indica
There are also numerous (over 2500) serovars within both species, which are found in a disparate variety of environments and which are associated with many different diseases. The vast majority of human isolates (>99.5%) are subspecies S. enterica. For the sake of simplicity, the CDC recommends that Salmonella species be referred to only by their genus and serovar, e.g.,
- Salmonella Typhi
instead of the more technically correct designation,
- Salmonella enterica subspecies enterica serovar Typhi.
Salmonella isolates are most commonly classified according to serology (Kauffman-White classification).[4] The main division is first by the somatic O antigen, then by flagellar H antigens. H antigens are further divided into phase 1 and phase 2. Both phase 1 and phase 2 H antigens are required for the full identification of an isolate but in practise, routine labs will leave this to Reference Laboratories.
Note that, with the exception of typhoid and paratyphoid, salmonellosis is not a blood-related infection, as is commonly believed.
Examples:
- Salmonella Enteritidis (1,9,12:g,m) - where the O antigens present are 1, 9 and 12; the H antigens are g and m.
- Salmonella Typhi (9,12,Vi:d:−) - where the O antigens are 9, 12,; the H antigen is d: The Vi antigen is associated with the bacterial capsule, which acts as a Virulence factor, hence its name.
In a clinical laboratory, only a small number of serovars are looked for (the remainder being rare or not clinically significant). The Health Protection Agency recommend testing for the following antigens routinely:
- O antigens: 2 4 6.7 8 9 and 3.10
- phase 1 H antigens: a b d E G i r Vi
- phase 2 H antigens: 1,2 1,5 1,6 1,7
Isolates that cannot be identified using this panel are sent to the reference laboratory for identification.
Tropism
Natural Reservoir
Related Chapters
- 1984 Rajneeshee bioterror attack
- List of foodborne illness outbreaks
- Food Testing Strips
References
- ↑ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0-8385-8529-9.
- ↑ Giannella RA (1996). "Salmonella". In Baron S et al (eds.). Baron's Medical Microbiology (4th ed. ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
- ↑ "Salmonella (Taxonomy)".
- ↑ 4.0 4.1 "The type species of the genus Salmonella Lignieres 1900 is Salmonella enterica (ex Kauffmann and Edwards 1952) Le Minor and Popoff 1987, with the type strain LT2T, and conservation of the epithet enterica in Salmonella enterica over all earlier epithets that may be applied to this species. Opinion 80". Int J Syst Evol Microbiol. 55 (Pt 1): 519–20. 2005. PMID 15653929.
- ↑ Tindall BJ; Grimont PAD, Garrity GM; Euzéby JP (2005). "Nomenclature and taxonomy of the genus Salmonella". Int J Syst Evol Microbiol. 55: 521&ndash, 524. PMID 15653930.