Gene Associations in Bacterial Pathogenesis: Pathogenicity Islands and Genomic Deletions

Abstract

This chapter examines how bacteria have altered their genome content to better survive and replicate within a disease environment. It specifically examines two of the latter evolutionary pathways and their relationship to the development of bacterial pathogenesis: (i) gene acquisition and (ii) gene deletion. Pathogenicity islands are perfect examples of how gene acquisition by horizontal transfer can generate sequence diversity in bacterial pathogens and enable them to evolve new virulence traits. As a pathogenic bacterium, Mycobacterium leprae can serve as a useful model in understanding the evolution of bacteria toward an obligate intracellular lifestyle. Researchers have elegantly reviewed the processes that contribute to the evolution of bacterial genomes, and their findings are used to summarize and bring perspective to the gene associations in bacterial pathogenesis. Pathogenicity islands and their associated genes, such as type III secretion systems, demonstrate how many bacterial species have acquired and evolved virulence functions using the same basic mechanism of horizontal gene transfer followed by mutation and adaptation to the lifestyle of the host bacterium. The insertion and deletion of pathogenicity islands as complete blocks of genes suggest that they may act as discrete molecular units within the genome of bacterial pathogens. Naturally competent bacterial species demonstrate many instances of sequence variation which have resulted from the recombination of small DNA fragments within genes. This provides another mechanism by which pathogens may evolve virulence functions with a good example being the evolution of penicillin-binding proteins in Streptococcus pneumoniae.