ABSTRACT
Typical virulent isolates of Yersinia pestis harbor three plasmids. One of these plasmids is ~70 kb in size and is also found in the other pathogenic Yersinia species, Yersinia pseudotuberculosis and Yersinia enterocolitica. This molecule is commonly referred to as pLcr (or pYV) and is discussed in detail in Chapter 16. The two other plasmids found in typical Y. pestis strains are specific for this species. The first is the ~100 kb plasmid generically designated pFra and the second is the ~9.6 kb element pPla. Both of these plasmids encode proteins unique to Y. pestis and factors necessary for full virulence of the organism. The fact that these molecules are Y. pestis-specific suggested that they might be involved in the highly invasive nature of the organism compared to the enteropathogenic Yersinia spp. (Brubaker, 1991). Genetic and virulence characterization of some of the genes encoded by these plasmids as well as virulence studies of natural isolates lacking pPla have demonstrated that virulence of this organism is not so simple. Certainly the acquisition of these plasmids was a major leap in the evolution of the
organism we now know as Y. pestis, however these elements do not completely account for the major increase in virulence of this species.
