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Salmonella

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This article is about the bacteria. For the disease, see Salmonellosis.


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Salmonella
Scientific classification
Kingdom: Bacteria
Class: Gamma Proteobacteria
Order: Enterobacteriales
Family: Enterobacteriaceae
Genus: Salmonella
Lignieres 1900
Species

S. bongori
S. enterica

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: a motile phase I and a nonmotile phase II. Cultures that are nonmotile upon primary culture may be switched to the motile phase using a Cragie tube.[citation needed]

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

Salmonella is properly pronounced /ˌsælməˈnɛlə/ voicing the initial letter "L," since it is named for pathologist D.E. Salmon, and has nothing to do with the salmon fish.

Contents

[edit] Salmonella as disease-causing agents

Salmonella infections are zoonotic and can be transferred between humans and nonhuman animals. Many infections are due to ingestion of contaminated food. A distinction is made between enteritis Salmonella and typhoid/paratyphoid Salmonella, where 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.

[edit] Enteritis Salmonella or Food Poisoning Salmonella

Constitutes a group consisting of potentially all other serotypes (over a thousand) of the Salmonella bacterium, most of which have never yet been found in humans. These are encountered in various Salmonella species, most having never been linked to a specific host, and can also infect humans. It is therefore a zoonotic disease. The organism enters through the digestive tract and must be ingested in large numbers to cause disease in healthy adults. Gastric acidity is responsible for the destruction of the majority of ingested bacteria. The infection usually occurs as a result of massive ingestion of foods in which the bacteria are highly concentrated similarly to a culture medium. However, infants and young children are much more susceptible to infection, easily achieved by ingesting a small number of bacteria. It has been shown that, in infants, the contamination could be through inhalation of bacteria-laden dust. After a short incubation period of a few hours to one day, the germ multiplies in the intestinal lumen causing an intestinal inflammation with diarrhea that is often muco-purulent and bloody. In infants, dehydration can cause a state of severe toxicosis. The disease usually is mild. There is normally no sepsis, but it can occur exceptionally as a complication in weakened elderly patients (Hodgkin's disease, eg.). Extraintestinal localizations are possible, especially Salmonella meningitis in children, osteitis, etc. Enteritis Salmonella (e.g., Salmonella enterica subsp. enterica serovar enteritidis) can cause diarrhea, which usually does not require antibiotic treatment. However, in people at risk such as infants, small children, the elderly, Salmonella infections can become very serious, leading to complications. If these are not treated, 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 [3]. 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.[4] According to the World Health Organization, over 16 million people worldwide are infected with typhoid fever each year, with 500,000 to 600,000 fatal cases.

Salmonella can survive for weeks outside a living body. They have been found in dried excrement after over 2.5 years.[citation needed] Salmonella are not destroyed by freezing.[citation needed] 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.[citation needed]

The AvrA toxin injected by the type three secretion system of Salmonella typhimurium works to inhibit the innate immune system by virtue of its serine/threonine acetyltransferase activity and requires binding to eukaryotic target cell phytic acid (IP6).[5] This leaves the host more susceptible to infection.

[edit] 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.[6] 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,[7] 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.

[edit] Salmonella nomenclature

Salmonella nomenclature is complicated. Initially each Salmonella species was named according to clinical considerations,[8] 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,[9] S. enterica, and the serovar were classified into six groups,[10] two of which are medically relevant. But as this now formalized nomenclature[11][12] 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).[13] Historically, serotype (V) was bongori, which is now considered its own species. The serovar classification of Salmonella is based on the Kauffman-White classification scheme that permits serological varieties to be differentiated from each other.

[edit] 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.

[edit] 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),
  • 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)(primarily aquatic turtles) is well described.[14]

Salmonella bacteria can survive several weeks in a dry environment and several months in water; thus, they are frequently found in polluted water, contamination from the excrement of carrier animals being particularly important. Aquatic vertebrates, notably birds and reptiles, are important vectors of salmonella. Poultry, cattle, and sheep frequently being agents of contamination, salmonella can be found in food, particularly meats and raw eggs.

File:Undercooked chicken.jpg
Improperly cooked poultry, such as the chicken seen here, may cause salmonella.

[edit] Deaths

About 142,000 Americans are infected each year with Salmonella enteritidis from chicken eggs, and about 30 die.[15] The shell of the egg may be contaminated by feces or environment, or its interior (yolk) may be contaminated in utero.

[edit] Medically relevant representatives

  • S. enterica subspecies arizonae, in cold-blooded animals, poultry, mammals
  • Serovars of S. enterica subspecies enterica
    • 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 septicemic salmonellosis in swine.
    • 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 A, solely a human pathogen, causes paratyphoid fever, transmission by contact and infected food or water
    • S. Paratyphi B, in central Europe usually a human pathogen, causes paratyphoid fever; transmission by contact and infected food, water or fly excrement
    • S. Typhi, occurs in temperate and subtropical zones, the human pathogen of typhoid fever; 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 wide range of infections in birds and mammals, ranging from self-limiting gastroenteritis to severe systemic paratyphoid diseases; 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
    • S. Brandenburg, causes abortion and enteritis in sheep and cattle

[edit] See also

[edit] References

  1. ^ Clark MA, Barret EL (June 1987). "The phs gene and hydrogen sulfide production by Salmonella typhimurium.". J Bacteriology 169 (6): 2391–2397. http://jb.asm.org/cgi/content/short/169/6/2391. 
  2. ^ Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 362–8. ISBN 0-8385-8529-9. 
  3. ^ § 6 and § 7 of the German law on infectious disease prevention, Infektionsschutzgesetz
  4. ^ Centers for Disease Control and Prevention
  5. ^ Mittal R, Peak-Chew SY, Sade RS, Vallis Y, McMahon HT (2010). "The acetyltransferase activity of the bacterial toxin YopJ of Yersinia is activated by eukaryotic host cell inositol hexakisphosphate". J Biol Chem 285 (26): 19927–34. doi:10.1074/jbc.M110.126581. PMID 20430892. PMC 2888404. http://www.jbc.org/content/early/2010/04/29/jbc.M110.126581.long. 
  6. ^ "FDA/CFSAN - Food Safety A to Z Reference Guide - Salmonella". FDA - Center for Food Safety and Applied Nutrition. 2008-07-03. http://www.cfsan.fda.gov/~dms/a2z-s.html. Retrieved 2009-02-14. 
  7. ^ http://www.cgmh.org.tw/chldhos/intr/c4a00/academy/bugs/salchole.html S. cholerasuis pathology. Accessed April 3., 2009
  8. ^ F. Kauffmann: Die Bakteriologie der Salmonella-Gruppe. Munksgaard, Kopenhagen, 1941
  9. ^ L. Le Minor, M. Y. Popoff: Request for an Opinion. Designation of Salmonella enterica. sp. nov., nom. rev., as the type and only species of the genus Salmonella. In: Int. J. Syst. Bacteriol., Bd. 37, 1987, S. 465–468
  10. ^ M. W. Reeves, G. M. Evins, A. A. Heiba, B. D. Plikaytis, J. J. Farmer III: Clonal nature of Salmonella typhi and its genetic relatedness to other salmonellae as shown by multilocus enzyme electrophoresis and proposal of Salmonella bongori comb. nov. In: J. Clin. Microbiol. Bd. 27, 1989, S. 313–320. PMID 2915026
  11. ^ Judicial Commission of the International Committee on Systematics of Prokaryotes: 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. In: Int. J. Syst. Evol. Microbiol. Bd. 55, 2005, S. 519–520. PMID 15653929
  12. ^ B. J. Tindall, P. A. Grimont, G. M. Garrity, J. P. Euzeby: Nomenclature and taxonomy of the genus Salmonella . In: Int. J. Syst. Evol. Microbiol. Bd. 55, 2005, S. 521–524. PMID 15653930
  13. ^ Janda JM, Abbott SL (2006). "The Enterobacteria", ASM Press.
  14. ^ "Ongoing investigation into reptile associated salmonella infections". Health Protection Report 3 (14). 9 April 2009. http://www.hpa.org.uk/hpr/archives/2009/news1409.htm#reptiles. Retrieved 12 April 2009. 
  15. ^ "Administration Urged to Boost Food Safety Efforts". Washington Post. 2009. http://www.washingtonpost.com/wp-dyn/content/article/2009/07/07/AR2009070702343.html?hpid=topnews. Retrieved 2009-07-07. "Among them is a final rule, issued by the FDA, to reduce the contamination in eggs. About 142,000 Americans are infected each year with Salmonella enteritidis from eggs, the result of an infected hen passing along the bacterium. About 30 die." 

[edit] External links

Wikimedia Commons has media related to: Salmonella
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