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

Abstract

SummaryThe distribution and the metabolic pathways of bacteria degrading steroid compounds released by eukaryotic organisms were investigated using the bile salt cholate as model substrate. Cholatedegrading 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 Pseudomonas sp. strain Chol1 releasing intermediates with a 3-keto-Δ 1,4 -diene structure of the steroid skeleton. The actinobacterium Dietzia sp. strain Chol2 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 Chol10-11, were isolated that produced the same cholate degradation intermediates as strain Chol2. With DOCDA as substrate for Pseudomonas sp. strain Chol1 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 Chol10-11. These results indicate that bacteria with potentially different pathways for cholate degradation coexist in natural habitats and may interact via interspecies cross-feeding.

An unexplored pathway for degradation of cholate requires a 7α‐hydroxysteroid dehydratase and contributes to a broad metabolic repertoire for the utilization of bile salts in Novosphingobium sp. strain Chol11

“…) according to our previous study (Holert et al . ). HATD has a 3‐keto‐Δ 1,4,6 ‐triene structure of the steroid ring showing that the elimination of water could also occur on steroids without side chain.…”

“…) and HDOCEA (7α‐hydroxy‐3,12‐dioxo‐4‐cholenoic acid, IV in Fig. ), respectively, according to our previous study (Holert et al ., ). These results suggested that Hsh2 is responsible for the dehydration of these steroid compounds.…”

“…In addition, HPLC‐MS analysis of supernatants from cultures grown with cholate, chenodeoxycholate and ursodeoxycholate revealed the transient accumulation of degradation metabolites with an absorption maximum at 290 nm indicative of steroid compounds with a 3‐keto‐Δ 4,6 ‐diene structure of the steroid skeleton. In supernatants of cultures grown with cholate, HOCDA and DOCDA could be identified according to our previous study (Holert et al ., ). In supernatants of cultures grown with chenodeoxycholate and ursodeoxycholate, two identical compound with a respective lower mass accumulated that matched Δ 4,6 ‐3‐keto‐lithocholate in both cases (Supporting Information Fig.…”

“…Bile salt degrading bacteria can be found amongst actinobacteria, in particular within the genus Rhodococcus (Mohn et al ., ). Among the proteobacteria, bile salt degrading strains occur within Alphaproteobacteria ( Sphingomonas , Novosphingobium (Holert et al ., )), Betaproteobacteria (especially Comamonas (Horinouchi et al ., ), Zoogloea (Holert et al ., )), and Gammaproteobacteria ( Pseudomonas (Holert et al ., ), Shewanella (Bergstrand et al ., )).…”

Sequence-based identification of inositol monophosphatase-like histidinol-phosphate phosphatases (HisN) in Corynebacterium glutamicum, Actinobacteria, and beyond

“…strain Chol2 [50], Zymomonas mobilis ZM4 [30], and Actinoplanes utahensis NRRL 12052 [31] was used for the amplification of hisN homologs from these bacteria.…”