Electron microscopic evidence for in vivo extracellular localization of Yersinia pseudotuberculosis harboring the pYV plasmid.

Electron microscopic evidence is presented that bacteria harboring the virulence plasmid pYV from Yersinia pseudotuberculosis are localized in extracellular sites during the course of infection in mice, often unambiguously undergoing active replication. Virulent pYV+ bacteria, often seen adherent to platelets, severely restricted granuloma formation, creating necrotic microabscesses poorly populated with inflammatory cells. This contrasts with granulomas produced by pYV- bacteria, which appear to be composed mainly of polymorphonuclear and mononuclear cells. Our results therefore strongly suggest that active replication of pYV+ bacteria predominantly, if not exclusively, occurs in vivo in extracellular sites.