PATHOGENICITY ISLANDS OF VIRULENT BACTERIA - STRUCTURE, FUNCTION AND IMPACT ON MICROBIAL EVOLUTION

Virulence genes of pathogenic bacteria, which code for toxins, adhesin s, invasins or other virulence factors, may be located on transmissibl e genetic elements such as transposons, plasmids or bacteriophages. In addition, such genes may be part of particular regions on the bacteri al chromosome, termed 'pathogenicity islands' (Pais). Pathogenicity is lands are found in Gram-negative as well as in Gram-positive bacteria. They are present in the genome of pathogenic strains of a given speci es but absent or only rarely present in those of non-pathogenic varian ts 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 bacteria l genome. In most cases, Pais are flanked by specific DNA sequences, s uch as direct repeats or insertion sequence (IS) elements. In addition , Pais of certain bacteria (e.g. uropathogenic Escherichia cell, Yersi nia spp., Helicobacter pylori) have the tendency to delete with high f requencies or may undergo duplications and amplifications. Pais are of ten associated with tRNA loci, which may represent target sites for th e chromosomal integration of these elements. Bacteriophage attachment sites and cryptic genes on Pais, which are homologous to phage integra se genes, plasmid origins of replication or IS elements, indicate that these particular genetic elements were previously able to spread amon g bacterial populations by horizontal gene transfer, a process known t o contribute to microbial evolution.