Virulence-plasmid is associated with the inhibition of opsonization in Yersinia enterocolitica and Yersinia pseudotuberculosis.

Plasmid-cured variants of virulent strains of Yersinia enterocolitica and Y. pseudotuberculosis were obtained by selection after growth in calcium-deficient medium. To obtain antigen preparations consisting of whole bacteria the original plasmid-containing strains and the plasmid-cured variants were grown in conditions favouring expression of the temperature-inducible outer membrane proteins of Yersinia (YOP) (37 degrees C, calcium-deficient culture medium). The presence or absence of the YOP on the bacteria was verified by immunoblotting. Opsonophagocytosis of YOP-negative Yersinia preparations (YOP-) was compared to that of YOP-containing ones (YOP+) in human polymorphonuclear leukocyte (PMN) chemiluminescence (CL) assay. The attachment of complement C3b on the surface of the bacteria after opsonization with normal human serum was determined by using a fluorescent anti-C3c-antibody and flow cytometry. YOP+ bacteria resisted opsonization in the absence of specific antibodies, as indicated by diminished C3b-fixation on bacteria and weaker CL response. This implies that virulence-plasmid-coded structures provide Y. enterocolitica and Y. pseudotuberculosis with an ability to avoid complement-mediated opsonization and phagocytosis.