Bacteriophages are important modulators of microbial communities. The mechanisms by which phages are maintained and shape bacterial competition remains to be fully characterized. A species-specific lytic bacteriophage was isolated to investigate their role in determining community composition and metabolic function. This study was conducted both in media and in gnotobiotic mice.These findings underscore the delicate balance between phage and microbial communities which dictate microbiome function in the gastrointestinal tract.