INTRODUCTION

Bacterial genomes are no longer considered a stable and rigid DNA structure carrying the essential genetic information for survival, fitness, and transmission of the species. With the development of genomic high-throughput techniques such as sequencing of whole bacterial genomes (with a total in November 2007 of 597 complete microbial genomes that are sequenced and 879 genomes in progress; http://www.ncbi.nlm.nih.gov/genomes/lproks.cgi) and the bioinformatics tools, our view of bacterial genome plasticity has changed. Bacterial genomes are now regarded as highly flexible and dynamic structures that change in size, genetic content, and organization over time (Hanage et al., 2006). This flexibility is also known as genome evolution (Groisman and Casadesus, 2005). It is thought that this evolution is driven by the adaptation of microorganisms to environmental niches, changes, or stresses and occurs via several mechanisms including point mutations, deletions, and gene acquisition/loss via horizontal gene transfer (Albiger et al., 1999; Chen et al., 2005). This latter process is associated with mobile genetic elements such as conjugative plasmids, bacteriophages, transposons, insertion sequence (IS) elements, and genomic islands (Albiger et al., 1999; Frost et al., 2005).

In Gram-negative bacteria, pathogenicity islands (PAI) are genomic regions that harbor one or more gene clusters encoding virulence-associated properties (reviewed in Gal-Mor and Finlay, 2006; Schmidt and Hensel, 2004). They are present in the genomes of pathogenic bacteria and usually absent from the same or closely related non-pathogenic species.