Salmonella

This page primarily focuses upon the bacteria. For the disease, see Salmonellosis

Salmonella is a genus of rod-shaped Gram-negative enterobacteria that causes typhoid fever, paratyphoid fever and foodborne illness. It is motile in nature and produces hydrogen sulfide.

History

Salmonella was named after Daniel Elmer Salmon, an American veterinary pathologist who, together with Theobald Smith (better known for his work on anaphylaxis), first discovered the Salmonella bacterium in 1885 from pigs. [http://213.236.152.40/www-whonamedit-com/doctor.cfm/408.html. Most cases involve undercooked meat, particularly poultry; other sources have been implicated in Salmonella enterica infections. Blood culture should be undertaken in cases where enteric fever, caused by Salmonella typhi or Salmonella paratyphi is suspected.

Microbiology

Salmonella are Gram-negative bacteria. In a clinical laboratory, they are usually isolated on MacConkey agar, XLD agar or DCA 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 practised. 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 salmonellas to become enriched in numbers. Salmonellas 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. They do not ferment lactose.

Classification

Salmonella taxonomy is complicated. }}^, }} Currently (7 Dec 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 (totalling 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 recommend that Salmonella species be referred to only by their genus and serovar:
e.g.,
Salmonella typhi
instead of the more correct designation,
Salmonella enterica subspecies enterica serovar Typhi.

Salmonella isolates are most usually classified according to serology (Kauffman-White classification). 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. The full description of a salmonella isolate is given as (O antigens, Vi : H antigen phase 1: H antigen phase 2).

Note that, with the exception of typhoid and paratyphoid, salmonella is not a blood-related infection, as is commonly believed.

Examples:

'Salmonella' Enteritidis (1,9,12:g,m)

(The O antigens present are 1, 9 and 12; the H antigens are g and m)

'Salmonella' Typhi (9,12,Vi:d:−)

(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: 1 2 3 4 5 6 7
phase 2 H antigens: a b c d E G i r

Isolates that cannot be identified using this panel are sent to the reference laboratory for identification.

Salmonella-associated diseases

Disease-causing salmonellas have recently been re-classified into a single species, Salmonella enterica, which has numerous strains or serovars. Salmonella Typhi is a well known serovar that causes typhoid fever. Other salmonellae are frequent causes of foodborne illness, and can especially be caught from poultry and raw eggs and more generally from food that has been cooked or frozen, and not eaten straight away. It can also be caught by handling reptiles, such as iguanas or terrapins, which commonly host the Salmonella bacteria. In March 2006, The New York Times reported that the US government said that 16.3% of all chickens were contaminated with salmonella. In the mid to late 20th century, Salmonella enterica serovar Enteritidis was a common contaminant of eggs. This is much less common now with the advent of hygiene measures in egg production and the vaccination of laying hens to prevent salmonella colonisation. Many different salmonella serovars also cause severe diseases in animals other than human beings.

After bacterial infections, Reiters Syndrome can develop.

References

External links