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CLASSIFICATION

First isolated in 1933 this Gram-negative rod is somewhat unique in that it is motile below 30ºC but not at 37ºC. It produces oval or coccoid cells in young cultures at 25ºC. Until the early 1970's Y. enterocolitica was thought to be a single species with much variation in its properties. However the biochemically atypical strains have now been classified as seven additional species within the Y. enterocolitica group: Y. aldovae, Y. bercovieri, Y. frederiksenii, Y. intermedia, Y. mollareti, Y. kristensenii and Y. rohdei.

BIOCHEMISTRY

Y. enterocolitica grows optimally at room temperature and continues to multiply at refrigerated temperatures. It produces colonies of 1.0 mm or less on nutrient agar, is oxidase negative, ferments glucose with little or no gas, lacks phenylalanine deaminase, is urease positive, and is unique as a pathogen in being psychrotrophic.

Y. enterocolitica exhibits 10 and 30 percent DNA homology with other genera of the Enterobacteriaceae and is approximately 50 percent related to Y. pestis and Y. pseudotuberculosis.

Serotyping of Y. enterocolitica is based on heat stable O antigens. Despite the usefulness of serotyping in predicting potential virulence, the scheme is flawed because of the presence of antigens in other bacteria which cross-react with Y. enterocolitica. Complete antigenic characterisation of isolates by O and H serotyping is rarely attempted. Other schemes used to subtype Yersinia species include bacteriophage typing, isoenzyme electropherotyping and the demonstration of restriction fragment length polymorphism of chromosomal and plasmid DNA. These alternative methods are used rarely.

Primary infection of intestinal tissue appears to proceed from adhesion to and invasion of the epithelial cells lining the intestinal lumen. The presence of specific chromosomal loci, inv and ail has been associated with pathogenicity. Elevated levels of iron in patients have been found to significantly enhance virulence of Y. enterocolitica. A heat-stable enterotoxin is produced by most clinical isolates; it is thought that the production of enterotoxin is not critical to virulence.

NORMAL FLORA

Y. enterocolitica is not part of the normal flora of humans. Non-pathogenic serotypes of Y. enterocolitica can be isolated from the faeces of healthy individuals.

Only certain serotypes of Y. enterocolitica are pathogenic. Serotypes that are typically pathogenic for humans are O:3; O:5, 27; O:8; and O:9. Infection: Infections with Y. enterocolitica are notable for their wide range of clinical presentations and outcomes. Most infections occur in children under the age of 5. Yersiniosis presents as diarrhea, often accompanied by low-grade fever and abdominal pain. In adolescents acute yersiniosis is often diagnosed as appendicitis. In addition to gastroenteritis Y. enterocolitica has also been associated with mesenteric lymphadenitis, terminal ileitis, reactive arthritis, peritonitis, colon and neck abscesses, cholecystis and erythema nodosum.

Suspect colonies can be identified as Yersinia by biochemical reactions on two media, Kligier iron agar and Christsen's urea agar, which are incubated at 35ºC. The species Y. enterocolitica can be identified by six additional tests: sucrose, Simmons citrate, raffinose, rhamnose, melibiose and a-methylglucoside, which are incubated between 22ºC and 25ºC. A low-calcium agarose medium containing Congo red may facilitate the detection of virulent, plasmid carrying strains. An enzyme immunoassay test has been developed to differentiate between pathogenic and non-pathogenic isolates.

Y. enterocolitica occupies a broad range of environments and has been isolated from the intestinal tracts of many different mammalian species as well as from birds, frogs, flies, fish, fleas, crabs and oysters. Foods that may harbour Y. enterocolitica include pork, beef, lamb and poultry. Y. enterocolitica is commonly found in a variety of ecosystems, including soil, vegetation, lakes, rivers, wells and streams.

None

An effective means of preventing yersiniosis is to educate the public in the proper handling and cooking of food. There are no specific vaccines available, the use of antibiotics may shorten the course of infection.

Enterobacteriaceae

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