finlay2006_PAIs - Microreview Pathogenicity islands: a...

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Unformatted text preview: Microreview Pathogenicity islands: a molecular toolbox for bacterial virulence Ohad Gal-Mor and B. Brett Finlay* Michael Smith Laboratories, University of British Columbia, Vancouver, BC, Canada. Summary Pathogenicity islands (PAIs) are distinct genetic ele- ments on the chromosomes of a large number of bacterial pathogens. PAIs encode various virulence factors and are normally absent from non-pathogenic strains of the same or closely related species. PAIs are considered to be a subclass of genomic islands that are acquired by horizontal gene transfer via transduction, conjugation and transformation, and provide quantum leaps in microbial evolution. Data based on numerous sequenced bacterial genomes demonstrate that PAIs are present in a wide range of both Gram-positive and Gram-negative bacterial pathogens of humans, animals and plants. Recent research focused on PAIs has not only led to the identification of many novel virulence factors used by these species during infection of their respective hosts, but also dramatically changed our way of thinking about the evolution of bacterial virulence. Nothing in biology makes sense except in the light of evolution. Theodosius Dobzhansky (19001975) Pathogens are characterized by defined differences from their non-pathogenic relatives: they have evolved the ability to cause disease in other organisms. In the follow- ing discussion, it is important to acknowledge that every microorganism has adapted to a particular niche, and pathogens are no exception. Pathogenicity represents another bacterial lifestyle, with the host serving merely as an additional ecological niche. As such, the evolutionary forces driving the adaptation of microorganisms to envi- ronmental niches function the same way in the evolution of pathogens. Thus, as it is true for all microbial genomes, bacterial pathogens have evolved by three major pro- cesses: (i) modification of existing genes, (ii) loss of genes no longer under selection, and (iii) gain of genes that confer benefit in their current ecological niche. In this review we will discuss various aspects of the third process, by focusing on the molecular and evolutionary mechanisms that lead to the acquisition and formation of pathogenicity islands and their contribution to bacterial virulence. Pathogenicity islands The first published complete microbial genome sequence of Haemophilus influenzae in 1995 (Fleischmann et al ., 1995) initiated a new era in bacterial pathogen evolution- ary research. To date, the complete sequence of more than 300 bacterial genomes has been published and the sequencing of more than 940 other bacterial genomes is currently ongoing (http://www.genomesonline.org/). Com- prehensive analyses of the sequence data have sug- gested that the genes encoding virulence functions, in many pathogens, were somehow different from other genes in the chromosome. Differences often include changes in overall nucleotide composition, codon usage bias, association with mobile genetic elements, and...
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finlay2006_PAIs - Microreview Pathogenicity islands: a...

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