Virulence genes of pathogenic bacteria, which code for toxins, adhesins, invasins or other virulence factors, may be located on transmissible genetic elements such as transposons, plasmids or bacteriophages. In addition, such genes may be part of particular regions on the bacterial chromosome, termed ¿pathogenicity islands¿ (PAIs). Pathogenicity islands are found in Gram-negative as well as in Gram-positive bacteria. They are present in the genome of pathogenic strains of a given species but absent or only rarely present in those of non-pathogenic variants of the same or related species. They comprise large DNA regions (up to 200 kb of DNA) and often carry more than one virulence gene, the G+C contents of which often differ from those of the remaining bacterial genome. In most cases, PAIs are flanked by specific DNA sequences, such as direct repeats or insertion sequence (IS) elements. In addition, PAIs of certain bacteria (e.g. uropathogenic Escherichia coli, Yersinia spp., Helico- bacfer pylori) have the tendency to delete with high frequencies or may undergo duplication and amplifications.