The bile acid pool, which is synthesized collaboratively by the host and its microbiome, impacts metabolism, immunity, and disease risk. Targeted microbiome interventions could in principle reshape the bile acid pool for therapeutic benefit, but practical strategies remain elusive. In the course of screening a complex defined community for metabolic phenotypes by dropping out individual strains, we observed that several of the single-strain dropout communities had markedly increased deoxycholic and lithocholic acid levels and a larger bile acid pool. In each of these communities, a second strain--Lactobacillus plantarum--had bloomed. The bile salt hydrolase activity of L. plantarum was necessary and sufficient to expand the size of the bile acid pool. An engineered community in which the bsh gene is overexpressed in multiple Lactobacillus strains confers on mice increased levels of secondary bile acid levels and a larger pool size. By overexpressing a different pair of bile acid metabolic genes in multiple strains of Lactobacillus--7a- and 7b-hydroxysteroid dehydrogenase--we changed the composition of the bile acid pool, enlarging it and redirecting it toward ursodeoxycholic acid. Together, these results demonstrate that fine details of the microbiome's strain composition can have a substantial effect on bile acid metabolism, and that rational manipulation of the microbiome can alter the size and composition of the bile acid pool.
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