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.
1Food and Drug Administration.
2Bell of Atri, Inc.