Collaborative Metabolism: Gut Microbes Play a Key Role in Canine and Feline Bile Acid Metabolism

Bile acids, produced by the liver and secreted into the gastrointestinal tract, are dynamic molecules capable of impacting the overall health of dogs and cats in many contexts. Importantly, the gut microbiota metabolizes host primary bile acids into chemically distinct secondary bile acids. This review explores the emergence of new literature connecting microbial-derived bile acid metabolism to canine and feline health and disease. Moreover, this review highlights multi-omic methodologies for translational research as an area for continued growth in veterinary medicine aimed at accelerating microbiome science and medicine as it pertains to bile acid metabolism in dogs and cats.

Correlation between Peptacetobacter hiranonis, the baiCD Gene, and Secondary Bile Acids in Dogs

Bile acid metabolism is a key pathway modulated by intestinal microbiota. Peptacetobacter (Clostridium) hiranonis has been described as the main species responsible for the conversion of primary into secondary fecal unconjugated bile acids (fUBA) in dogs. This multi-step biochemical pathway is encoded by the bile acid-inducible (bai) operon. We aimed to assess the correlation between P. hiranonis abundance, the abundance of one specific gene of the bai operon (baiCD), and secondary fUBA concentrations. In this retrospective study, 133 fecal samples were analyzed from 24 dogs. The abundances of P. hiranonis and baiCD were determined using qPCR. The concentration of fUBA was measured by gas chromatography-mass spectrometry. The baiCD abundance exhibited a strong positive correlation with secondary fUBA (ρ = 0.7377, 95% CI (0.6461, 0.8084), p < 0.0001). Similarly, there was a strong correlation between P. hiranonis and secondary fUBA (ρ = 0.6658, 95% CI (0.5555, 0.7532), p < 0.0001). Animals displaying conversion of fUBA and lacking P. hiranonis were not observed. These results suggest P. hiranonis is the main converter of primary to secondary bile acids in dogs.

The 7-α-dehydroxylation pathway: An integral component of gut bacterial bile acid metabolism and potential therapeutic target

The gut microbiome plays a significant role in maintaining host metabolic health through the production of metabolites. Comprising one of the most abundant and diverse forms of gut metabolites, bile acids play a key role in blood glucose regulation, insulin sensitivity, obesity, and energy expenditure. A central pathway in gut bacterial bile acid metabolism is the production of secondary bile acids via 7-ɑ-dehydroxylation. Despite the important role of 7-ɑ-dehydroxylation in gut bacterial bile acid metabolism and the pathophysiology of metabolic disease, the regulation of this pathway is not completely understood. This review aims to outline our current understanding of 7-ɑ-dehydroxylation and to identify key knowledge gaps that will be integral in further characterizing gut bacterial bile acid metabolism as a potential therapeutic target for treating metabolic dysregulation.

Comparative Genomic and Physiological Analysis against Clostridium scindens Reveals Eubacterium sp. c-25 as an Atypical Deoxycholic Acid Producer of the Human Gut Microbiota

The human gut houses bile acid 7α-dehydroxylating bacteria that produce secondary bile acids such as deoxycholic acid (DCA) from host-derived bile acids through enzymes encoded by the bai operon. While recent metagenomic studies suggest that these bacteria are highly diverse and abundant, very few DCA producers have been identified. Here, we investigated the physiology and determined the complete genome sequence of Eubacterium sp. c-25, a DCA producer that was isolated from human feces in the 1980s. Culture experiments showed a preference for neutral to slightly alkaline pH in both growth and DCA production. Genomic analyses revealed that c-25 is phylogenetically distinct from known DCA producers and possesses a multi-cluster arrangement of predicted bile-acid inducible (bai) genes that is considerably different from the typical bai operon structure. This arrangement is also found in other intestinal bacterial species, possibly indicative of unconfirmed 7α-dehydroxylation capabilities. Functionality of the predicted bai genes was supported by the induced expression of baiB, baiCD, and baiH in the presence of cholic acid substrate. Taken together, Eubacterium sp. c-25 is an atypical DCA producer with a novel bai gene cluster structure that may represent an unexplored genotype of DCA producers in the human gut.