ABSTRACT The gut microbiome is complex, raising questions about the role of individual strains in the community. Here, we address this question by focusing on a functional unit within the community, the metabolic niche that controls bile acid 7α-dehydroxylation. By constructing variants of a complex defined community in which we drop out strains that occupy this niche, we explore how interactions within and between niches shape community-level metabolism. Omitting both members of the niche, Clostridium scindens ( Cs ) and Clostridium hylemonae ( Ch ), eliminates secondary bile acid production and reshapes the community in a highly specific manner: eight strains go up or down in relative abundance by &gt;100-fold, while the remaining strains are largely unaffected. In single-strain dropout communities (i.e., a strain swap within the niche), Cs and Ch reach the same relative abundance and dehydroxylate bile acids to a similar extent. However, the effect on strains in other niches differs markedly: Clostridium sporogenes increases &gt;1000-fold in the Δ Cs but not Δ Ch dropout, reshaping the pool of microbiome-derived phenylalanine metabolites. Thus, strains that are functionally redundant within a niche can have widely varying impacts outside the niche, and a strain swap can ripple through the community in an unpredictable manner, resulting in a large impact on an unrelated community-level phenotype. Mice colonized by the Δ Cs Δ Ch community show decreased liver steatosis relative to those colonized by the Δ Ch community, demonstrating that a single strain from the microbiome can have a substantive impact on host physiology. Our work opens the door to the mechanistic studies of the role of an individual strain on community ecology and host physiology.

Strain dropouts reveal interactions that govern the metabolic output of the gut microbiome