Phages to shape the gut microbiota?

• A diverse collection of bacteria and phages stably coexists within our gut. • Gut dysbiosis seems to be marked by altered bacterial and phage composition. • Exogenous phages, including whole virome transplant, alter gut bacteria populations. • Novel intestinal phage biology can inform phage therapy approaches. The concept of (bacterio)phage therapy is simple; target the phage to the bacterial pathogen causing disease. As phages are natural killers of bacteria, one could expect this to be an easy task. However, when it comes to phage therapy within the gut, it might not be quite that simple. Already without exogenous intervention, a multitude of phage–bacterial interactions occur within the human gut, some of which might play a direct role in disease progression. In this perspective, we aim to summarise the current understanding of phages within our gut, moving from infancy, adulthood, and then into disease progression. We then highlight recent advances in phage-based interventions, both conventional phage therapy and the progressing field of whole virome transplant.