The unusual convergence of steroid catabolic pathways in Mycobacterium abscessus

Mycobacterium abscessus, an opportunistic pathogen responsible for pulmonary infections, contains genes predicted to encode two steroid catabolic pathways: a cholesterol catabolic pathway similar to that of Mycobacterium tuberculosis and a 4-androstenedione (4-AD) catabolic pathway. Consistent with this prediction, M. abscessus grew on both steroids. In contrast to M. tuberculosis, Rhodococcus jostii RHA1, and other Actinobacteria, the cholesterol and 4-AD catabolic gene clusters of the M. abscessus complex lack genes encoding HsaD, the meta-cleavage product (MCP) hydrolase. However, M. abscessus ATCC 19977 harbors two hsaD homologs elsewhere in its genome. Only one of the encoded enzymes detectably transformed steroid metabolites. Among tested substrates, HsaD(Mab) and HsaD(Mtb) of M. tuberculosis had highest substrate specificities for MCPs with partially degraded side chains thioesterified with coenzyme A (k(cat)/K-M = 1.9 x 10(4) and 5.7 x 10(3) mM(-1)s(-1), respectively). Consistent with a dual role in cholesterol and 4-AD catabolism, HsaD(Mab) also transformed nonthioesterified substrates efficiently, and a Delta hsaD mutant of M. abscessus grew on neither steroid. Interestingly, both steroids prevented growth of the mutant on acetate. The Delta hsaD mutant of M. abscessus excreted cholesterol metabolites with a fully degraded side chain, while the corresponding RHA1 mutant excreted metabolites with partially degraded side chains. Finally, the Delta hsaD mutant was not viable in macrophages. Overall, our data establish that the cholesterol and 4-AD catabolic pathways of M. abscessus are unique in that they converge upstream of where this occurs in characterized steroid-catabolizing bacteria. The data further indicate that cholesterol is a substrate for intracellular bacteria and that cholesterol-dependent toxicity is not strictly dependent on coenzyme A sequestration.

Automated summary

Mycobacterium abscessus contains genes predicted to encode two steroid catabolic pathways, and unlike other similar bacteria, it lacks genes encoding MCP hydrolase. The study shows that cholesterol is essential for the growth of the bacteria and the catabolic pathways are not solely dependent on coenzyme A sequestration.