Foodborne diseases / Yersinia enterocolitica

Yersinia enterocolitica

Yersinia enterocolitica, which can be isolated from many species of mammals and birds, is an important cause of gastroenteritis in humans, especially in temperate countries. An important property of the bacterium is its ability to multiply at temperatures near to 0°C, and hence on chilled foods.

1. Bacteriological characteristics

Y. enterocolitica is a Gram-negative, facultatively anaerobic rod and belongs to the family Enterobacteriaceae. Y. enterocolitica can be divided into over 70 types. The geographical distribution of the various pathogenic serotypes is different. O:3 and O:9 are the most important serotypes in Europe, and O:3, particularly, in large areas of the world. Serotypes O:8 and O:5,27 have so far dominated in the USA. In recent years, serotype O:3 has been on the increase in the USA.

2. Disease in humans

Gastroenteritis is by far the most common symptom of yersiniosis in humans. The clinical picture is usually one of a self-limiting diarrhoea associated with mild fever and abdominal pain. Infections are, in some patients, followed by reactive arthritis or reactive skin complaints, erythema nodosum being the most common. The minimum infectious dose required to cause disease is unknown while the incubation period is estimated at 2-11 days.

3. Sources of infection and epidemiology

Healthy pigs are often carriers of strains of Y. enterocolitica that are pathogenic to humans, in particular strains of O:3 and O:9. The organisms are present in the oral cavity, especially the tongue and tonsils, and in the intestine and faeces, Healthy pigs have been found to be infected with Y. enterocolitica O:3 in frequencies up to 85%. Strains of O:3 have been found frequently on the surfaces of freshly slaughtered pig carcasses (in frequencies up to 63%). In contrast to the frequent occurrence of the bacterium in pigs and on freshly slaughtered carcasses, pathogenic Y. enterocolitica have only exceptionally been found on retail pork products, with the exception of fresh tongues. This phenomenon might be explained by the lack of proper selective methods for the isolation of pathogenic strains. The association between yersiniosis in humans and the consumption of raw pork in Belgium, and the apparently low prevalence of the infection in Moslem countries, where pork consumption is low, point to pork as a source of infection with Y. enterocolitica. The following variables were found to be independently related to an increased risk of yersiniosis in a case-control study conducted in Norway: drinking untreated water, general preference for meat to be prepared raw or undercooked, and frequency of consumption of pork and sausages.
Outbreaks of yersiniosis have often been linked to recontaminated pasteurised milk and milk products while direct spread among humans has also been reported.

4. Growth properties in relation to meat and meat products

The reported growth range is -2 to 42°C with an optimum temperature of 28-29°C.
Y. enterocolitica can multiply at temperatures approaching and even below 0°C. The organism is not able to grow at pH <4.2 or >9.0 or at water activities below 0.945. The ability to propagate at refrigeration temperatures in long-life vacuum-packed foods is of considerable significance in food hygiene. Y. enterocolitica may survive in frozen foods for long periods. The organism does not survive pasteurization or normal cooking (boiling, baking, frying) temperatures. Heat treatment at 60°C for 1-3 minutes effectively inactivates Y. enterocolitica. Under anaerobic conditions, Y. enterocolitica is unable to grow on beef at pH 5.4-5.8, where growth occurs at pH 6.0. an atmosphere of 100% carbon dioxide inhibits the growth of Y. enterocolitica at <5°C and pH 6.0.

5. Isolation

Isolation of Y. enterocolitica may involve pre-enrichment in a non-selective buffered medium incubated at a low temperature (4-25°C) followed by plate culture or selective enrichment in either a modified Rappaport broth or a bile-oxalate-sorbose broth incubated at 25°C. Direct selective enrichment in irgasan-ticarcillin-potassium chlorate (ITC) enrichment broth is another time saving procedure. The selective agars used include cefsulodin-irgasan-novobiocin (CIN) agar or Salmonella-Shigella agar with 1% sodium deoxycholate and 0.1% CaCl2 (SSDC). Genetic probes can also be used in colony hybridization to demonstrate pathogenic Y. enterocolitica strains. Immunomagnetic separation (IMS) methods and polymerase chain reaction (PCR) procedures have also been developed.