Potential Role of Refrigerated Milk Packaging in the Transmission of Listeriosis and Salmonellosis.

Cultures of three Listeria monocytogenes serotypes and three Salmonella spp. were applied to the exterior surfaces of waxed cardboard or plastic milk containers. Contamination sites were sampled with premoistened cotton swabs during 14 d of refrigeration. Unstressed cells of Listeria survived up to 14 d on the surfaces of waxed (1 serotype) and plastic (3 serotypes) containers. Heat-stressed cells of all three serotypes of Listeria survived for 2 d on both types of containers. One serotype survived for 4 d, but only on plastic containers. Unstressed cells of all three Salmonella strains survived up to 14 d on both types of containers. Heat-stressed Salmonella strains survived up to 2 d (waxed containers) and 4 d (plastic containers).

Yersinia enterocolitica : Survival of a Pathogenic Strain on Milk Containers.

A strain of Yersinia enterocolitica isolated from a patient in a milk-associated yersiniosis outbreak in Tennessee, Mississippi and Arkansas in the summer of 1982 was used to contaminate the exterior of refrigerated milk containers. The bacteria survived on the containers for as long as 21 d, as shown by recovery on MacConkey agar plates or in veal infusion broth. Y. enterocolitica was not detected in milk poured from the contaminated containers.

Yersiniosis Associated with Tofu Consumption: Serological, Biochemical and Pathogenicity Studies of Yersinia enterocolitica Isolates.

From December 1981 to February 1982, 87 individuals (ages two months to 74 years) in the Seattle, WA, area developed the clinical symptoms of yersiniosis. Illness was related to consumption of commercial tofu (soybean curd) contaminated with Yersinia enterocolitica . The six Y. enterocolitica strains recovered from the hospitalized patients indicated that two antigenically distinct strains, 0:8 and 0:Tacoma, were involved. At the manufacturing site of the incriminated tofu, 112 Y. enterocolitica strains were recovered, of which two were serotype 0:8. The reactions of these strains were similar to those of clinical 0:8 strains in biochemical tests and in eight virulence factor tests. The LD50 for suckling mice was identical for all strains which killed mice. Although the causative organism(s) was not recovered from other samples of packaged tofu, our findings incriminated water used in the processing of tofu as the source of infection. The source of the second Y. enterocolitica strain (0:Tacoma) in this outbreak was not identified.

Pathogenicity of Yersinia enterocolitica Demonstrated in the Suckling Mouse.

An experimental suckling mouse intraperitoneal injection test was compared with four plasmid-associated tests (adult mouse peroral exposure, adult mouse intraperitoneal injection, auto-agglutination and plasmid detection by gel electrophoresis) to measure Yersinia enterocolitica pathogenicity. Of eight Vwa plasmid-harboring strains (O:3; O:4,32; O:5,27; O:8; O:9; O:13; O:21; and O:Tacoma) and one isogenic plasmidless strain (O:8), all Vwa plasmid-harboring strains gave identical results in all tests except the two adult mouse tests. In studies of 35 clinical strains of Y. enterocolitica recently isolated during two foodborne outbreaks, a comparison of the autoagglutination, gel electrophoresis for Vwa plasmid detection and suckling mouse tests showed that 29 strains (83%) gave identical results in all three tests. The other six strains produced different reactions in the plasmid detection and autoagglutination tests, indicating that neither test alone is sufficient to evaluate the virulence of Y. enterocolitica . To compare the sensitivity of these in vitro tests with a biological assay (the suckling mouse intraperitoneal injection test), a mixture of plasmid-harboring (P+) and plasmidless (P-) isogenic Y. enterocolitica cells was examined. The suckling mouse test was more sensitive and consistent in detecting the Vwa plasmid (as evidenced by mouse lethality). A bacterial population containing 0.1 % P + cells induced a lethal infection in the suckling mouse, whereas the other two tests required at least 10% P + cells for detection of the Vwa plasmid. The 50% lethal dose (LD50) in the suckling mouse was directly proportional to the number of Vwa-harboring cells in the culture.