Simple method for demonstration of differential colony morphology of plasmid-associated virulent clones of Yersinia enterocolitica.

Virulent (plasmid-associated) strains of Yersinia enterocolitica grown on RPMI 1640 agar (RPMI-1640 medium [Flow Laboratories] with 40 mM HEPES and 1.5% [wt/vol] purified agar [Difco]) dissociated into small and large colonies. The autoagglutination test (a marker of virulence) is regularly positive with small colonies and negative with large colonies. Avirulent Y. enterocolitica strains gave only large colonies on RPMI 1640 agar.