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Thirty-nine strains of Yersinia enterocolitica and 10 strains of Yersinia pseudotuberculosis were studied for the presence of fimbriae, plasmids and two plasmid associated phenotypic expressions, autoagglutination and Ca2+ dependent growth at 37 degrees C. All of the strains studied became fimbriated, which was confirmed by electron microscopy and haemagglutination tests. Fimbriation was not correlated with the presence or absence of plasmids.

In Y. enterocolitica strain, serovar 0:10, the capacity for the formation of pili inducing the mannose-resistant hemagglutination (MRHA) of formolated sheep red blood cells was due to the presence of plasmid pYE10. MRHA-inducing pili differed serologically from Y. pestis and Y. tuberculosis adhesion pili. Plasmid pYE10 was immobilized for transfer to cells of Escherichia coli strain HB101 (rec A) by means of pRP 4. The expression of MRHA-inducing pili in the new host the rec A-independent character of the synthesis. Y. enterocolitica cells containing pYE10 agglutinated in tissue-culture media with 10% of serum added at 37 degrees C.