Lifestyle evolution in symbiotic bacteria: insights from genomics

Bacteria that live only in eukaryotic cells and tissues, including chronic pathogens and mutualistic bacteriocyte associates, often possess a distinct ive set of genomic traits, including reduced genome size, biased nucleotide base composition and fast polypeptide evolution. These phylogenetically di verse bacteria have lost certain functional categories of genes, including DNA repair genes, which affect mutational patterns. However, pathogens and mutualistic symbionts retain loci that underlie their unique interaction ty pes, such as genes enabling nutrient provisioning by mutualistic bacteria-i nhabiting animals, Recent genomic studies suggest that many of these bacter ia are irreversibly specialized, precluding shifts between pathogenesis and mutualism.