The Three Bears and Virulence-Associated Plasmids in the Genus Yersinia

The Yersiniae are Gram-negative rods belonging to the family Enterobacteriaceae and represent a useful genus to study the evolution of virulence embodied in their plasmid DNA content (Wren 2003). Yersinia species range from the benign to the highly virulent, exemplified by the plague bacillus Yersinia pestis whose effects during the Black Death wiped out a third of the European population and helped to shape Western civilization. Acquisition of plasmids is central to the development of virulence in the Yersiniae. All three human pathogenic species including Y. pestis and the enteropathogenic Yersiniae, Y. pseudotuberculosis, and Y. enterocolitica, harbor a 70-kb virulence plasmid (pYV), which is essential for infection in lymph tissues as well as to overcome host defense mechanisms. Additionally, Y. pestis has two further plasmids, a large (100-kb pMT1) and a small plasmid (9.6-kb pPla), which are essential for transmission via the flea vector (summarized in Table 1); these two plasmids are absent from Y. pseudotuberculosis and Y. enterocolitica. To date, sequencing has been performed on plasmids isolated from several strains of Y. pestis and Y. pseudotuberculosis and one strain of Y. enterocolitica (Table 2). This chapter will describe the salient features of these plasmids named after the three bears, daddy plasmid, baby plasmid, and the mother of all plasmids, pYV.