Aerobic and Anaerobic Biotransformation of Bile Acids by Escherichia coli (I)

The oxidation/reduction reactions of bile acids by Escherichia coli (E. coli) K-12 were examined in both Davis and brain-heart infusion (BHI) media under aerobic and anaerobic conditions. The pH in the Davis medium changed by almost the same amount, around pH 6.5–7.0 in both aerobic and anaerobic cultures, but the pH in the BHI medium was different in both cultures, that is, about pH 9.0 in the aerobic culture but only about 6.5 in the anaerobic culture. The growth curve of E. coli in the Davis medium showed a similar pattern in both conditions. Cholic acid (CA) was oxidized to 3α12αdihydroxy-7-oxo-5β-cholanoic acid (3α12α7=O) in both cultures, but the reaction in the anaerobic culture was somewhat slower than that in the aerobic culture. On the other hand, reduction of 3α12α7=O to CA did not occur in the aerobic culture, but about 10% reduction was observed in the anaerobic culture after 4 days. These data suggest that the oxidation/reduction reaction of E. coli was oxidative in aerobic culture but reductive in anaerobic culture and these characteristics were not due to the changes in the pH of the medium. The reactions of CA and glycocholic acid to crude 7α-HSDH prepared from E. coli were examined and it was found that both free and conjugated CA as a substrate for the 7α-HSDH showed similar Km values.