Pathogenicity Islands and Their Role in Bacterial Virulence and Survival

Abstract

Infections caused by bacterial pathogens are still a significant problem in modern medicine. Therefore, the identification of the factors that are related to the infections and the understanding of the processes involved in the evolution of pathogenic bacteria from their nonpathogenic progenitors is an important subject of research. It has long been known that acquisition of virulence determinants by horizontal gene transfer is one of the major driving forces in the emergence and evolution of new pathogens [reviewed in 1–4]. Furthermore, our knowledge of the organization of the bacterial genome has greatly increased within the last few years due to the availability of more than 120 completely sequenced eubacterial genomes, including those of almost all pathogenic bacteria, which has introduced a new area of pathogen research. It has become evident that the typical bacterial genome consists of a conserved ‘core gene pool’ comprising genes that encode essential structural features and fundamental metabolic pathways, and a ‘flexible gene pool’ that is more variable and encodes functions only advantageous under specific growth conditions. Core genes are characterized by a relatively homogenous G C content and they are normally encoded in stable regions of the chromosome that are conserved in their organization in closely related species. In contrast, the flexible gene pool comprises variable regions of the chromosome and various mobile genetic elements such as plasmids, bacteriophages, IS elements and transposons, conjugative transposons, integrons and superintegrons that are transferred between different organisms by the means of natural transformation, transduction or conjugation. Many of the genes encoding toxins, adhesins, secretion systems, invasins or other virulence-associated factors have been found to be encoded by mobile genetic elements [overviews in 5, 6]. Furthermore, the analysis of the genomes