Enterococci have emerged as leading causes of multidrug resistant hospital infection. Yet they have existed as commensal microbes of everything from insects to man for over 400 million years. Work in the initial period of support showed that multidrug resistant hospital strains have swollen genomes about 25% larger than those of commensal strains. This additional DNA consists largely of a pathogenicity island (PAI) and other fitness islands, antibiotic resistance and other plasmids, transposons, IS elements and phage. We found that the multidrug resistant hospital strains with swollen genomes that emerged in the 70's and later lacked an important defense of the fidelity of the chromosome - the CRISPR locus. Moreover, we showed that in pheromone plasmid containing strains with multiple copies of IS elements, all chromosomal genes could be transferred to a recipient by an Hfr- like plasmid mobilization mechanism. The goal of the competing continuation period is to understand the mechanisms underlying the plasticity and movement of a pathogenicity island in enterococci - the largest mobile element described in the species, and one that harbors most of the know virulence traits. Specifically, data from the initial support period suggested that the E. faecalis PAI evolved by the accretion of modules, possibly mediated by transposases or other site specific recombinases found in the PAI. We now propose to test the integrity and stability of these modules, and determine how genes occurring within a module are functionally linked in disease and colonization environments. Moreover, we propose to determine whether the PAI site specific recombinases are vestigial, or whether they continue to serve an important role in the dissemination of the PAI. These studies will illuminate the mechanisms responsible for the emergence of enterococci as increasingly virulent, multidrug resistant hospital pathogens from roots as benign commensals. Moreover, by understanding the mechanisms involved, we will have the information necessary to quantify the risk of spread of resistance and virulence features among enterococci, and the information needed to understand how widespread antibiotic use selected for microbes with enhanced ability to acquire and accumulate resistance as well as virulence traits.