Role of a Plasmid in the Pathogenicity of Yersinia Species

Abstract

Plasmids are self-replicating extrachromosomal elements found in bacteria of many genera (Broda 1979). They range in size from approximately 2 to 500 kilobases (kb). Thus, the genetic coding capacity of these plasmids may vary from one or two polypeptides to several hundred polypeptide chains. They may, therefore, impart considerable genetic information to plasmid-bearing strains. In most instances plasmids are dispensable for the growth of bacteria in the laboratory environment. Plasmids, however, often carry genes whose expression provide bacteria with a selective advantage in certain adverse environments. During the past decade, it has become apparent that plasmids frequently encode determinants of bacterial pathogenicity (Elwell and Shipley 1980). The most thoroughly examined example of the latter type of plasmids is those encountered in the enterotoxigenic Escherichia coli, whose two major determinants of pathogenicity, colonization factors (which afford bacteria the ability to adhere to host cells) and toxin production, are plasmid-encoded traits (Elwell and Shipley 1980). In addition, all of the invasive pathogenic members of the Enterobac- teriaceae have been shown to have a plasmid which is essential for pathogenicity. Invasive E. coli strains often harbor plasmids of the colicin V class. These plasmids code for either an iron-sequestering system and/or resistance to the bactericidal action of normal human serum (Binns et al. 1979; Williams and Warner 1980). Shigella flexneri strains carry a large plasmid whose expression appears necessary for inducing keratoconjunctivitis in guinea pigs and invasion of HeLa cell monolayers (Sansonetti et al. 1982).