Selection of Mycobacterium sp. strains with capacity to biotransform high concentrations of beta-sitosterol

In this work, phytosterol-biotransforming strains were selected from Mycobacterium sp., using a high concentration of beta-sitosterol. The selection was made by culturing the strains in a medium enriched with 14 g beta-sitosterol/l as the unique source of carbon. During 2 months, the bacterial cultures were transferred successively. The extraction of the biotransformation products was made with methanol and ethyl acetate. The qualitative and quantitative analysis was made by means of thin-layer chromatography, gas-liquid chromatography (GLC) and GLC-mass spectrometry. Under these conditions, it was observed that after seven transfers, the strains MYcobacterium sp. MB-3683 and the Mycobacterium fortuitum B-11045 increased their biotransformation capacity from 20% to 64% and from 34% to 55%, respectively. The products in the highest proportion identified for each trial were androstenedione and androstadienedione. The results suggest that the high substrate concentration could be a selective mechanism to obtain strains more efficient in the biotransformation of beta-sitosterol into steroidal bases.