Role of virulence-associated plasmid in the uptake and killing of Yersinia pseudotuberculosis by resident macrophages.
Resistance to phagocytosis of Yersinia pseudotuberculosis by mouse resident macrophages was investigated using a highly virulent plasmid-bearing strain and its low-virulent plasmid-free derivative. It was found that plasmid-bearing and plasmid-less bacteria did not resist the ingestion step nor inhibit the oxidative burst and phagolysosomal fusion in the peritoneal macrophages. These in vitro data were further confirmed by the study of bacterial survival in infected mice. Bacteria from plasmid-bearing and plasmid-free strains were similarly eliminated during the first hours of infection in the liver and in the peritoneal cavity. Our results therefore suggest that plasmid-mediated virulence is not related to the resistance to the bactericidal activity of fixed macrophages of the reticuloendothelial system during the early phase of infection, and that plasmid-encoded factors are expressed after a latency period by an unknown mechanism.