A synthetic gut microbiota provides an understanding of the maintenance and functional impact of phage

Abstract

Phages are under intense study as therapeutics and mediators of microbial community behavior; however, tractable models are needed to study phages in the context of the mammalian gut. To address this gap, we isolated phages against members of a synthetic gut microbial community (sFMT), identifying the Bacteroides uniformis JEB00023 (DSM 6597) phage HKP09. While resistance to HKP09 was observable within hours of infection in monoculture, high titers of HKP09 were maintained in vitro and in gnotobiotic mouse models over extended periods. Sequencing of resistant B. uniformis lines revealed phase variation upstream of a capsular polysaccharide locus driving the generation of resistant and sensitive subpopulations, thus demonstrating a mechanism allowing stable coexistence of both virus and bacterial host. Communities infected in vitro and in vivo with HKP09 showed transiently reduced loads of B. uniformis DSM 6597. Its impact in the gut was distinct from communities constructed without its host B. uniformis strain (sFMT∆JEB00023). Rather than a compensatory increase in closely related Bacteroides strains, the most significant impacts were observed on distantly related strains, demonstrating that phage perturbations more broadly impact community structure in ways not easily predicted by phylogeny or simple strain exclusion. Metabolomic analyses of the feces of HKP09-infected sFMT-colonized gnotobiotic animals demonstrated altered abundances of amino acids and microbial fermentation products compared to uninfected mice and those colonized with sFMT∆JEB00023. Taken together, these data provide a controlled model for studying phages in the context of the mammalian gut, providing mechanistic insights into phage-host dynamics and their consequences on the function of microbial communities.

Importance: Phages are key members of the gut microbiome, but the understanding of their biological significance for host health lags behind their bacterial hosts. In this study, we demonstrate the use of a phage-infection model using defined, synthetic microbial communities that colonize the intestinal tract of mice. We uncovered that spontaneous inversions in the genome of Bacteroides uniformis perpetually generate subpopulations, which are either sensitive or resistant to phage infection, allowing for the coexistence of predator and prey in this species. Phage infection demonstrated broad impacts on community structure and metabolism in animals, which are not easily predicted by the exclusion of the viral host. This research demonstrates a tractable approach through which the impacts of phage on both the microbiome and mammalian host can be deciphered.

Keywords: bacteriophages; gnotobiotics; gut microbiome; metagenomics; synthetic communities.