Comparison of three tests for virulent Yersinia enterocolitica.

Magnesium oxalate agar was found to select against virulent types of Yersinia enterocolitica. Small colonies isolated from magnesium oxalate agar which were presumably virulent generally contained no detectable virulence-specific plasmids, did not agglutinate with virulence-specific antiserum, and exhibited various degrees of virulence in mice. Results indicate that the virulence potential of a Y. enterocolitica isolate cannot always be identified by these in vitro methods and suggest that virulence also involves other factors yet to be identified.