The Concept of Pathogenicity Islands

Abstract

In the early 1980s it was discovered that chromosomal regions may carry blocks of virulence-associated genes and may differ between related members of certain species or genera. These regions were termed pathogenicity islands (PAIs). The basic observation leading to the concept of PAIs was the finding that particular genomic regions of pathogens carry virulence-associated genes together with loci whose presence strongly indicates horizontal gene transfer of these regions between different species or even genera. PAIs occur in the genomes of various pathogens with the capacity to cause infections not only in humans but also in animals and even in plants. The list of PAIs described up to now includes those in bacteria for which frequent gene transfer via plasmids, bacteriophages, and conjugative transposons has been described. The existence of PAIs in eukaryotic pathogens can be predicted, because gene transfer also exists in eukaryotic organisms, transposable elements frequently occur, and retroviruses have a tendency to integrate into tRNA genes. It is now accepted that the generation of PAIs often starts with the integration of plasmids, phages, conjugative transposons, or cointegrates of these into specific target genes, preferentially on the chromosomes.