Evidence of distinct pathways for bacterial degradation of the steroid compound cholate suggests the potential for metabolic interactions by interspecies cross‐feeding

Summary The distribution and the metabolic pathways of bacteria degrading steroid compounds released by eukaryotic organisms were investigated using the bile salt cholate as model substrate. Cholate‐degrading bacteria could be readily isolated from freshwater environments. All isolated strains transiently released steroid degradation intermediates into culture supernatants before their further degradation. Cholate degradation could be initiated via two different reaction sequences. Most strains degraded cholate via a reaction sequence known from the model organism P seudomonas sp. strain Chol 1 releasing intermediates with a 3‐keto‐Δ 1,4 ‐diene structure of the steroid skeleton. The actinobacterium D ietzia sp. strain Chol 2 degraded cholate via a different and yet unexplored reaction sequence releasing intermediates with a 3‐keto‐Δ 4,6 ‐diene‐7‐deoxy structure of the steroid skeleton such as 3,12‐dioxo‐4,6‐choldienoic acid ( DOCDA ). Using DOCDA as substrate, two Alphaproteobacteria , strains Chol 10–11, were isolated that produced the same cholate degradation intermediates as strain Chol 2. With DOCDA as substrate for P seudomonas sp. strain Chol 1 only the side chain was degraded while the ring system was transformed into novel steroid compounds accumulating as dead‐end metabolites. These metabolites could be degraded by the DOCDA ‐producing strains Chol 10–11. These results indicate that bacteria with potentially different pathways for cholate degradation coexist in natural habitats and may interact via interspecies cross‐feeding.