Cell, Vol. 87, 791±794, November 29, 1996, Copyright 1996 by Cell Press Pathogenicity Islands: Minireview Bacterial Evolution in Quantum Leaps Eduardo A. Groisman* and Howard Ochman² for the secretion of virulence proteins by Yersinia, Erwinia, *Department of Molecular Microbiology Pseudomonas, Xanthomonas, Shigella, and Salmonella Washington University School of Medicine (Van Gijsegem et al., 1993)Ðand these systems differ from St. Louis, Missouri 63110 the sec-dependent pathway in that their effector mole- ² Department of Biology cules lack a typical signal sequence. University of Rochester At the same location where LEE has inserted into entero- Rochester, New York 14627 pathogenic strains (McDaniel et al., 1995) a distinct patho- genicity island, PAI-1, has been identified in a uropatho- ªI am the beneficiary of a lucky break in the genetic genic strain of E. coli (Blum et al., 1994; Figure 1). Both sweepstakesº 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; see Figure 1), can be of any length, but, most often, they accommodate large clusters of genes contributing a particular virulence phenotype. Pathogenicity islands have been recognized by their association with transmissible DNA, their sporadic distribution in a set of related pathogenic and non-patho- genic organisms, or the unusual features of their encoded genes, and their recent characterization has provided clues regarding their origins, distribution, mode of transfer, and genetic stability. Pathogenicity Islands Dictate Disease Condition Incorporation of a pathogenicity island can, in a single step, transform a normally benign organism into a patho- gen. For example, enteropathogenic E. coli contain a chro- mosomal segment that mediates attaching and effacing lesions on intestinal epithelial cells (McDaniel et al., 1995; Figure 1). This 35 kb regionÐtermed LEE for locus of en- terocyte effacementÐis absent from laboratory strains Figure 1. Location of Selected Pathogenicity Islands and Phages and codes for an outer membrane protein that promotes of Gram-Negative Bacteria contact with host cells and a type III secretion system that Chromosomes and pathogenicity islands are depicted as blue lines exports proteins known to modify host cell functions. Type and black triangles, respectively, and are not drawn to scale. The III secretion systems are prevalent among animal and plant presence of repeated sequences at the site of insertion, which is pathogensÐhomologous export systems are responsible shown below the island, is indicated by short yellow lines. Cell 792 different tRNA loci: the island at the pheV locus contains pathogen. The SPI-1 invasion island is present in represen- genes for P-fimbriae, which allow the pathogen to adhere tative strains from all subspecific groups of Salmonella, to cells lining the urinary tract, whereas the island at the although sporadic cases of secondary loss have been pheR locus codes for Prs fimbriae, which bind Gal Nac- reported for individual isolates of S. enterica sv. Seften- 1±3-Gal Nac±containing receptors rather the Gal-b-1±4- berg and Litchfield. But, in contrast to SPI-1, SPI-2- Gal residues recognized by the P fimbrial adhesins (Blum hybridizing sequences have not been detected in S. et al., 1995). In addition, the island at the pheR locus codes bongori (subspecies V), the most divergent lineage of for a type 1 cytotoxic necrotizing factor, which causes Salmonella. The incorporation of the SPI-1 and SPI-2 multinucleation and enlargement of eukaryotic cells and pathogenicity islands enabled Salmonella to invade host can trigger phagocytosis by epithelial cells (Falzano et al., cells, evade host defense systems, and cause systemic 1993). Sequences similar to those present in the pathoge- infections in mammals, while its close relative E. coli nicity islands of uropathogenic strains have been detected evolved as a commensal and opportunistic pathogen. in plasmids of hemolytic strains of E. coli, highlighting the On the other hand, the four nominal species of Shigella, mobile nature of these sequences. all of which emerged relatively recently from E. coli, Pathogenicity Islands Mediate Specific gained the ability to enter host cells following the acqui- Phases of the Infection Process sition of the large virulence plasmid, thereby becoming Two pathogenicity islands have been identified in Salmo- invasive pathogens of primates. nella, each contributing to a specific step in the course Gain and Loss of Pathogenicity Islands of infection. The island at 63', designated SPI-1 (Figure The occurrence of pathogenicity islands raises several 1), governs the ability of Salmonella to invade epithe- questions about the evolution of virulence in bacteria. lial cells (Mills et al., 1995; Gala n, 1996), and a second First, what advantage does the incorporation of viru- island at 31', designated SPI-2, mediates survival within lence genes into the chromosome provide over retaining macrophages (Ochman et al., 1996; Shea et al., 1996). them on episomes? Although many virulence determi- Both islands are 40 kb in length and appear to have nants occur on extrachromosomal DNAs, integration been acquired by horizontal gene transfer because their into the chromosome circumvents the need for autono- G1C contents are lower than those typical of Salmonella mously replicating elements. This also lowers the proba- (Groisman et al., 1993). bility that a given gene will be eliminated because epi- The nucleotide sequence of the SPI-1 island is nearly somes must be under continuous selective pressure to complete, and this region includes at least 25 genes, the be maintained whereas the spontaneous elimination of majority of which encode components of a Type III secre- chromosomal genes is not as high. However, plasmid tion system and its effector proteins (Gala n, 1996). The integration can result in the inactivation of chromosomal assemblage of genes specifying this export apparatus in loci or in the alteration of the expression of plasmid- Salmonella is similar in size, sequence, and organization encoded genes: for example, insertion of the Shigella to a set of genes in the Shigella virulence plasmid (Grois- flexneri virulence plasmid into the metB locus leads to man and Ochman, 1993), and several of the proteins en- methionine auxotrophy and decreased expression of coded by these gene clusters arecapable of cross-species the invasion proteins (Zagaglia et al., 1991). complementation. However, certain genes within the Sal- Second, how are pathogenicity islands incorporated monella invasion island, such as orgA and hil,haveno into the bacterial chromosome? Analysis of the junc- counterparts in the Shigella virulence plasmid and may be tions of pathogenicity islands can reveal the mechanism responsible for the differential regulation of invasion in used to integrate these sequences. As noted above, the Shigella and Salmonella: this property is controlled by genes encoded by PAI-1 and LEE of E. coli are clearly temperature in Shigella and by oxygen tension, osmolarity, different, but the site of insertion within the selC-tRNA and pH in Salmonella. The vast differences in G1C con- gene is conserved (McDaniel et al., 1995). This suggests tents and the phylogenetic distribution of the invasion that both islands were acquired by a similar mechanism gene clusters argue that Salmonella and Shigella gained which was likely to be phage-mediated since selC also these sequences