High-level production of Tauroursodeoxycholic acid from Taurochenodeoxycholic acid in engineered Bacillus subtilis via whole-cell biotransformation.

Bear bile powder, a valuable traditional Chinese medicinal material, is distinguished by its high content of tauroursodeoxycholic acid (TUDCA), typically present at a ratio of 0.82 to 1.76 relative to taurochenodeoxycholic acid (TCDCA). In this study, the generally recognized as safe (GRAS) bacterium Bacillus subtilis was engineered to create a two-step enzymatic pathway for TCDCA to TUDCA bioconversion, utilizing bidirectional redox enzymes 7α/7β-hydroxysteroid dehydrogenase. Ribosome binding site (RBS) optimization and glucose 1-dehydrogenase (GDH)-mediated NAD(P)H cofactor regeneration were implemented to increase TUDCA production. The engineered strains A2-2 and A2-2-5 produced 15.10 ± 0.36 g/L and 14.33 ± 1.24 g/L of TUDCA, with TUDCA/TCDCA ratios of 1.22 and 0.88, respectively, following 15 h of fermentation in a whole-cell biocatalytic system. T-7-KLCA intermediate was undetectable. This study provided a cost-effective and scalable approach for the industrial production of potential bear bile substitutes from readily available chicken bile powder using an engineered probiotic B. subtilis microbial cell factory.