Influence of a 70 kilobase virulence plasmid on the ability of Yersinia enterocolitica to survive phagocytosis in vitro.

During the course of infection, Yersinia enterocolitica invades tissues where macrophages and polymorphonuclear leucocytes (PMNs) constitute the first line of defence. As expression of virulence in Y. enterocolitica is governed in part by a c. 70 kilobase virulence plasmid (pYV), we investigated the influence of this plasmid on the interaction between Y. enterocolitica and phagocytes in vitro. The results showed that, irrespective of plasmid-carriage, yersiniae survived phagocytosis by macrophages and PMNs. Plasmidless Y. enterocolitica that had grown intracellularly in macrophages, however, were susceptible to killing by PMNs, whereas plasmid-bearing bacteria were resistant. In vitro cultivation of Y. enterocolitica in a Ca(2+)-deficient medium resembling that found within macrophages, did not influence the susceptibility of plasmid-bearing and plasmidless strains to killing by PMNs. These results indicate that passage through macrophages renders plasmidless strains of Y. enterocolitica susceptible to killing by PMNs. This finding may explain some of the differences in the behaviour of plasmid-bearing and plasmidless strains of Yersinia species in vivo.