The mammalian intestine is home to a dense community of bacteria and its associated bacteriophage (phage). Virtually nothing is known about how phages impact the establishment and maintenance of resident bacterial communities in the intestine. Here, we examine the phages harbored by Enterococcus faecalis, a commensal of the human intestine. We show that E. faecalis strain V583 produces a composite phage (ϕV1/7) derived from two distinct chromosomally encoded prophage elements. One prophage, prophage 1 (ϕV1), encodes the structural genes necessary for phage particle production. Another prophage, prophage 7 (ϕV7), is required for phage infection of susceptible host bacteria. Production of ϕV1/7 is controlled, in part, by nutrient availability, because ϕV1/7 particle numbers are elevated by free amino acids in culture and during growth in the mouse intestine. ϕV1/7 confers an advantage to E. faecalis V583 during competition with other E. faecalis strains in vitro and in vivo. Thus, we propose that E. faecalis V583 uses phage particles to establish and maintain dominance of its intestinal niche in the presence of closely related competing strains. Our findings indicate that bacteriophages can impact the dynamics of bacterial colonization in the mammalian intestinal ecosystem.T he human gastrointestinal tract is colonized with a highly diverse population of bacteria (1). Many of these bacteria produce bacteriophage (phage), further increasing the complexity of this ecosystem. Recent studies of human intestinal viromes have shown that these viromes are dominated by lysogenic prophages that are integrated into the chromosomes of their bacterial hosts (2). In numerous other ecological systems, phages profoundly influence ecological networks by serving as reservoirs of genetic diversity (3, 4) and acting as predators of susceptible bacterial strains (5). However, little is known about how resident phages may influence the assembly and maintenance of bacterial communities in the mammalian intestine.Enterococcus faecalis is an abundant member of the human intestinal microflora, and by adulthood, it can constitute as much as 0.5-0.9% of the total bacterial content of the intestinal tract (Table S1) (6). A Gram-positive facultative anaerobe, E. faecalis is a leading cause of antibiotic-resistant nosocomial bacteremia and endocarditis (7). Genomic sequencing has revealed a high degree of variation among E. faecalis genomes (8). Some of this variation can be attributed to an array of integrated prophage elements that encode components required for the production of phage particles.The first E. faecalis genome to be sequenced was strain V583, a clinical blood isolate that is vancomycin-resistant (9). The V583 chromosome harbors seven putative prophages designated prophages 1-7 (Fig. 1). At least two of these prophage elements seem to encode cryptic or satellite phage genomes (10) that, by themselves, do not produce functional phage particles but may encode accessory components that aid in the lytic cycle of other integrated pro...