Microbial Oxidation of the Fusidic Acid Side Chain by Cunninghamella echinulata

Biotransformation of fusidic acid (1) was accomplished using a battery of microorganisms including Cunninghamella echinulata NRRL 1382, which converted fusidic acid (1) into three new metabolites 2–4 and the known metabolite 5. These metabolites were identified using 1D and 2D NMR and HRESI-FTMS data. Structural assignment of the compounds was supported via computation of (1)H- and (13)C-NMR chemical shifts. Compounds 2 and 3 were assigned as the 27-hydroxy and 26-hydroxy derivatives of fusidic acid, respectively. Subsequent oxidation of 3 afforded aldehyde 4 and the dicarboxylic acid 5. Compounds 2, 4 and 5 were screened for antimicrobial activity against different Gram positive and negative bacteria, Mycobacterium smegmatis, M. intercellulare and Candida albicans. The compounds showed lower activity compared to fusidic acid against the tested strains. Molecular docking studies were carried out to assist the structural assignments and predict the binding modes of the metabolites.