Bacteria referred to as Y. enterocofitica and Y. enterocolitica-like organisms are quite heterogenous with respect to phenotypic, genotypic and ecological properties. This bacterial group encompassesacknowledged pathogens as well as a range of so-called environmental strains which are ubiquitous in terrestrial and freshwater ecosystems (Kapperud and Bergan, 1984; Mollaret et al., 1979). Strains previously termed 'Y. enterocolitica-like organisms' have now been classified as seven separate species: Y. frederikseniL Y. kristenseniL }I. intermedia, Y. aldovae, Y. rohdei, Y. mollaretii and Y.bercovieri (Aleksic et al., 1987; Bercovier et al., 1984; Brenner et al., 1980; Wauters et al., 1988b). None of these species have been associated with human or animal disease, with the exception of a few atypical cases. All strains capable of causing disease (yersiniosis) belong to Y. enterocolitica sensu stricto. However, not all strains of this species are pathogenic (Mollaret et al., 1979).The phenotypic heterogeniety in Y. enterocolitica has prompted the development of several schemes for routine subdivision of the bacterium on the basis of serogroups, biovars, and phagevars (Kapperud and Bergan, 1984: Mollaret et al., 1979; Wauters et al., 1987). Y. enterocolitica has been classified into approx. 60 serogroups on the basis of O antigens (Wauters, 1981). However, the strainsassociated with disease in man or animals belong to only a few serogroups. Thus, 0:3, 0:5,27, 0:8, and 0:9 are the most important causative agents in man. A correlation exists between geographic areas and the serogroups associated with disease in man (Mollaret et al., 1979; Fig. 1). The serogroups most frequently causing human yersiniosis in Europe are 0:3 followed by 0:9.
Differentiation betweenpathogens and non-pathogens
Accordingly, a number of different in vitro and in vivo tests have been proposed for differentiation of pathogenic and non-pathogenic variants (Prpic et al., 1985; Robins-Browne et al., 1989; Schiemann, 1989; Wachsmuth et al., 1984). These tests include: autoagglutination at 37°C, calcium-dependent growth restriction at 37°C, resistance to the bactericidal activity ofnormal human serum, various animal and cell culture models, binding of Congo Red dye, use of DNA probes, and biochemical parameters. Many of these tests are based on properties associated with a 40-50-MDa plasmid which is a prerequisite, but not sufficient in itself, for virulence within the genus Yersinia (Cornelis et al., 1987; Portnoy and Martinez, 1985).
Clinical manifestationsGastroenteritis is by far the most frequently encountered manifestation. Acute non-complicated enteritis is observed most often in children below 7 years of age. Occasionally, the infection is confined to the right iliac fossa in the form of acute terminal ileitis or acute mesenteric lymphadenitis, giving rise to.symptoms resembling appendicitis. Post-infection manifestations, including arthritis anderythema nodosum, occur mainly in adults, the latter most commonly in women. The incubation period of Y. enterocolitica enterocolitis ranges from 1 to 11 days. The minimal infective dose for humans has not been determined (Cover and Aber, 1989).
Growth and survival in food
Y. enterocolitica is able to multiply at temperatures approaching 0°C, a circumstance which means that it can grow in...