MICROBIOLOGY :SALMONELLA

Salmonella is a genus of rod-shaped, Gram-negative, non-spore forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which project in all directions (i.e. peritrichous). They are chemoorganotrophs, obtaining their energy from oxidation and reduction reactions using organic sources and are facultative anaerobes; most species produce hydrogen sulfide,[1] which can readily be detected by growing them on media containing ferrous sulfate, such as TSI. Most isolates exist in two phases; phase I is the motile phase and phase II the non-motile phase. Cultures that are non-motile upon primary culture may be swithched to the motile phase using a Craigie tube.

Salmonella are closely related to the Escherichia genus and are found worldwide in warm- and cold-blooded animals, in humans, and in nonliving habitats. They cause illnesses in humans and many animals, such as typhoid fever, paratyphoid fever, and the foodborne illness salmonellosis.[2]

Salmonella is named for pathologist D.E. Salmon.

Contents
1 Salmonella as disease-causing agent
2 History
3 Salmonella nomenclature
4 Genetics
5 Sources of infection
6 Deaths
7 Medically relevant representatives
8 See also
9 References
Salmonella as disease-causing agent
Salmonella infections are zoonotic; they can be transmitted by humans to animals and vice versa. Infection via food is also possible. A distinction is made between enteritis salmonella and typhoid/paratyphoid salmonella, whereby the latter because of a special virulence factor and a capsule protein (virulence antigen) can cause serious illness, such as Salmonella enterica subsp. enterica Serovar Typhi, or Salmonella typhi). Salmonella typhi is adapted to humans and does not occur in animals.

Enteritis Salmonella (e.g., Salmonella enterica subsp. enterica serovar Enteritidis) can cause diarrhoea, which usually does not require antibiotic treatment. However, people at risk such as infants, small children, the elderly, HIV patients and those with suppressed immunity can become seriously ill. Children with sickle cell anemia who are infected with salmonella may develop osteomyelitis.

In Germany, Salmonella infections must be reported (§ 6 and § 7 of the German law on infectious disease prevention, Infektionsschutzgesetz). Between 1990 and 2005 the number of officially recorded cases decreased from approximately 200,000 cases to approximately 50,000. It is estimated that every fifth person in Germany is a carrier of Salmonella. In the USA, there are approximately 40,000 cases of Salmonella infection reported each year.[3] According to the World Health Organization, over 16 million people worldwide are infected with typhoid fever each year, with 500,000 to 600,000 of these cases proving to be fatal.

Salmonella can survive for weeks outside a living body. They have been found in dried excrement after over 2.5 years. Ultraviolet radiation and heat accelerate their demise; they perish after being heated to 55 °C (131 °F) for one hour, or to 60 °C (140 °F) for half an hour.[citation needed] To protect against Salmonella infection, it is recommended that food be heated for at least ten minutes at 75 °C (167 °F) so that the center of the food reaches this temperature. Salmonella is not destroyed by freezing.


History
The genus Salmonella was named after Daniel Elmer Salmon, an American veterinary pathologist. While Theobald Smith was the actual discoverer of the type bacterium (Salmonella enterica var. Choleraesuis) in 1885, Dr. Salmon was the administrator of the USDA research program, and thus the organism was named after him.[4] Smith and Salmon had been searching for the cause of common hog cholera and proposed this organism as the causal agent. Later research, however, would show that this organism (now known as Salmonella enterica) rarely causes enteric symptoms in pigs,[5] and was thus not the agent they were seeking (which was eventually shown to be a virus). However, related bacteria in the genus Salmonella were eventually shown to cause other important infectious diseases.


Salmonella nomenclature
Salmonella nomenclature is complicated. Initially each Salmonella species was named according to clinical considerations,[6] e.g., Salmonella typhi-murium (mouse typhoid fever), S. cholerae-suis (hog cholera). After it was recognized that host specificity did not exist for many species, new strains (or serovar, short for serological variants) received species names according to the location at which the new strain was isolated. Later, molecular findings led to the hypothesis that Salmonella consisted of only one species,[7] S. enteric, and the serovar were classified into six groups,[8] two of which are medically relevant. But as this now formalized nomenclature[9][10] is not in harmony with the traditional usage familiar to specialists in microbiology and infectologists, the traditional nomenclature is common. Currently, there are two recognized species: S. enterica and S. bongori, with six main subspecies: enterica (I), salamae (II), arizonae (IIIa), diarizonae (IIIb), houtenae (IV), and indica (VI).[11] Historically, serotype (V) was bongori, which is now considered its own species.


Genetics
Serovar Typhimurium has considerable diversity and may be very old. The majority of the isolates belong to a single clonal complex. Isolates are divided into phage types, but some phage types do not have a single origin as determined using mutational changes. Phage type DT104 is heterogeneous and represented in multiple sequence types, with its multidrug-resistant variant being the most successful and causing epidemics in many parts of the world.

Serovar Typhi is relatively young compared to Typhimurium, and probably originated approximately 30,000-50,000 years ago.


Sources of infection
Unclean food, particularly in institutional kitchens and restaurants,
Excretions from either sick or infected but apparently clinically healthy people and animals (especially endangered are caregivers and animals),and is of considerable girth
Polluted surface water and standing water (such as in shower hoses or unused water dispensers),
Unhygienically thawed fowl (the meltwater contains many bacteria),
An association with reptiles (pet tortoises and snakes) is well described.[12]


Deaths
About 142,000 Americans are infected each year with Salmonella enteritidis from chicken eggs and about 30 die.[13]


Medically relevant representatives
S. enterica ssp. arizonae, in cold-blooded animals, poultry, mammals
S. choleraesuis (Bacillus paratyphoid B and C), intestinal commensalists in pigs, pathogenic if resistance is weak; humans can be infected by ingesting sick animals; the bacteria cause hog cholera
S. enteritidis, in the intestines of cattle, rodents, ducks (and their eggs) and humans; causes calf paratyphoid fever and acute gastroenteritis in humans
S. paratyphi
S. paratyphi A, solely a human pathogen, causes paratyphoid A, transmission by contact and infected food or water
S. paratyphi B, in central Europe usually a human pathogen, causes paratyphoid B; transmission by contact and infected food, water or fly excrement
S. typhi, occurs in temperate and subtropical zones, the human pathogen of typhus abdominalis; transmission by contact and infected food, water or fly excrement; 3–5 % of all persons falling ill remain permanent carriers of the pathogen
S. typhimurium, causes a usually fatal, feverish intestinal infection in birds and mammals; conveyed by contaminated foodstuffs; causes Salmonella enteritis ("food poisoning") in humans
S. dublin, one of the pathogens causing cattle salmonellosis
S. typhisuis, one of the pathogens causing hog salmonellosis