Pathogenicity Islands: Bacterial Evolution in Quantum Leaps

(Van Gijsegem et al., 1993)—and these systems differ from the sec-dependent pathway in that their effector moleSt. Louis, Missouri 63110 † Department of Biology cules lack a typical signal sequence. At the same location where LEE has inserted into entero-pathogenic strains (McDaniel et al., 1995) a distinct patho-genicity island, PAI-1, has been identified in a uropatho-" I am the beneficiary of a lucky break in the genetic genic strain of E. coli (Blum et al., 1994; Figure 1). Both sweepstakes " islands are targeted to the same basepair downstream of Isaac Asimov the selC gene, which codes for the selenocysteine-specific tRNA. LEE and PAI-1 contain different sets of genes: PAI-1 The Molecular Bases of Virulence is 70 kb in length and encodes a hemolysin. Thus, the For most bacterial pathogens virulence is a multifactorial specific cassette incorporated at the selC locus deter-process requiring two general classes of determinants. mines whether strains of E. coli are converted into entero-The first encompasses genes that participate in physiolog-or uropathogens. ical processes necessary for survival in host and non-host Aside from PAI-1 and LEE, three other pathogenicity environments, and these genes are generally found in both islands have been identified in E. coli (Figure 1); and, in pathogenic and non-pathogenicorganisms. In Salmonella, fact, certain uropathogenic strains contain more than one this set includes regulatory genes, such as phoP/phoQ, pathogenicity island. In addition to PAI-1, E. coli strain 536 that control expression of more than 20 loci in response also harbors a 190-kb pathogenicity island, PAI-2, that to low magnesium environments; biosynthetic genes, such has integrated at the tRNAleuX locus and contains genes as aroA, that encodes the ability to synthesize aromatic for an alternate hemolysin and Prf-type fimbriae (Blum et amino acids not found in host tissues; and several addi-al., 1994). The other pathogenicity islands detected in E. tional genes required for cell maintenance and DNA repair coli also contain hemolysin genes but are associated with (Groisman and Ochman, 1994). The second class of virulence genes specifies traits that are unique to pathogens, and, not surprisingly, these genes are rarely detected in non-pathogenic organisms. Based on the initial characterizations of plasmids from Yersinia and Shigella, such sequences were originally thought to be confined to extrachromosomal elements; but recently, several virulence cassettes have been mapped to the chromosome of pathogenic organisms. These segments of the chromosome, termed pathogenic-ity islands (Hacker et al., 1990; Lee, 1996; …