Salmonella

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]

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History

Salmonella was named after Daniel Elmer Salmon, an American veterinary pathologist, although it was his partner Theobald Smith (better known for his work on anaphylaxis) who first discovered the bacterium in 1885 from pigs.^[3][4]

Microbiology

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 complex 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.

Classification

Salmonella taxonomy is complicated.^[5],[6] 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).^[5] 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.

Prevention

The prevention of Salmonella as a food illness involves effective sanitizing of food contact surfaces. Alcohol has proven to be an effective topical sanitizer against Salmonella. Quaternary ammonium can be used in conjunction with alcohol as a food-contact safe sanitizer with increased duration of the sanitizing action. Nonflammable Alcohol Vapor in carbon dioxide NAV-CO2 systems or sodium hypochlorite (bleach) are frequently used to sanitize surfaces to prevent Salmonella. Food containing raw eggs should be thoroughly cooked or frozen before eating.

Salmonella-associated diseases

Main article: Salmonellosis

Disease-causing Salmonella species have recently been re-classified into a single species, Salmonella enterica, which has numerous serovars. Salmonella Typhi causes typhoid fever. Other salmonellae are frequent causes of foodborne illness, especially from poultry and raw eggs and more generally from food that has been cooked or frozen and not eaten straight away. While these infections would normally only require a treatment of antibiotics, if not resolved naturally, the long-term usage of antibiotics in both the poultry and beef industries may have created a strain of salmonella which is potentially resistant to antibiotics.^[7]

Salmonellosis can also be caught by handling reptiles, such as iguanas or turtles. A CDC study also noted cases of salmonellosis in 2003 and 2004 associated with handling commercially distributed pet rodents. [1]

See also

• 1984 Rajneeshee bioterror attack
• List of foodborne illness outbreaks
• Food Testing Strips

References