Preservation of conjugated primary bile acids by oxygenation of the small intestinal microbiota in vitro

Authors: Firrman, Jenni; FRIEDMAN, ELLIOT - University Of Pennsylvania; HECHT, AARON - University Of Pennsylvania; STRANGE, WILLIAM - University Of Pennsylvania; Narrowe, Adrienne; Mahalak, Karley; WU, GARY - University Of Pennsylvania; Liu, Linshu

Submitted to: mBio
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2024
Publication Date: 5/10/2024
Citation: Firrman, J., Friedman, E.S., Hecht, A., Strange, W.C., Narrowe, A.B., Mahalak, K.K., Wu, G.D., Liu, L.S. 2024. Preservation of conjugated primary bile acids by oxygenation of the small intestinal microbiota in vitro. mBio. 15(6). https://doi.org/10.1128/mbio.00943-24.
DOI: https://doi.org/10.1128/mbio.00943-24

Interpretive Summary: Bile acids are released into the small intestine to help digest food into pieces that can be easily absorbed by the human cells. However, bile acids also can stop bacteria from multiplying. In response, the bacteria in the gastrointestinal tract have evolved so they are capable of converting bile acids into less active forms and, therefore, are less effective against them. This conversion of bile acids by the resident bacteria occurs rapidly in the human colon, but not in the small intestine. While this is beneficial to the human host, the reason why bacteria in the small intestine do not convert bile acids was unknown. To fill in this gap in knowledge, we developed a model of the small intestine in an in vitro, or artificial, system and used this to grow the small intestine bacteria from 4 separate people. We started this experiment using an environment with no oxygen and then added low levels of oxygen, similar to levels that would be found in a human. From the results of this experiment, we were first able to characterize the mixture of bacteria that grew, defining which bacteria were there and determining what they were doing. Next, we were able to detect and measure bile acids and found that the addition of oxygen inhibited the conversion of bile acids by the bacteria. These results are important because they showed, for the first time, that the presence of oxygen changed the ability for bacteria to perform conversion of bile acids and that oxygen plays a role in digestion in the gastrointestinal tract.

Technical Abstract: Bile acids play a critical role in emulsification of dietary lipids, a critical step in the primary function of the small intestine, which is the digestion and absorption of food. Primary bile acids delivered into the small intestine are conjugated to enhance functionality, in part, by increasing aqueous solubility and preventing passive diffusion of bile acids out of the gut lumen. Bile acid function can be disrupted by the gut microbiota through deconjugation of primary bile acids through expression of bile salt hydrolases (BSH) leading to their conversion into secondary bile acids by induction of bacterial bile acids inducible genes (BAI), a process often observed in malabsorption due to small intestinal bacterial overgrowth. By recapitulating the small intestinal microbiota in vitro using bioreactors inoculated with human ileostomy effluent, we show that the infusion of physiologically relevant levels of oxygen, normally found in the proximal small intestine, reduced deconjugation of primary bile acids, in part, through the expansion of bacterial taxa known to have low abundance of BSHs. Further recapitulating the small intestinal bile acid composition of the small intestine, there was no conversion of primary into secondary bile acids observed. Remarkably, these effects were preserved amongst four separate bioreactors, each inoculated with a different small intestinal microbiota, despite a high degree of taxonomic variability under both anoxic and aerobic conditions. In total, these results provide evidence for a previously unrecognized role that the oxygenated environment of the small intestine plays in the maintenance of normal digestive physiology.