Pathogenicity and Other Genomic Islands

Abstract

Horizontal gene transfer (HGT) has been increasingly recognized as a major contributor to genetic diversity, especially in bacteria. The genetic units that participate in HGT are known as mobile genetic elements (MGEs), many of which occupy specific places in the bacterial chromosome and are known as genomic islands (GEIs). A key feature of GEIs is that they never carry genes that are essential for the life cycle of the organism in which they reside; rather, they carry accessory genes that are involved in the adaptation of the organism to various environmental contingencies. Accordingly, GEIs have been classified on the basis of accessory functions. The first of these to be identified carry genes involved in the causation of disease – thus pathogenicity islands (PAIs). Others since discovered have been classified as resistance islands, metabolic islands, and symbiosis islands. Hence, although this article is focused on the PAIs, it is emphasized that this designation, though useful in the study of bacterial pathogenesis, is artificial and its usage will probably decline.