Chrysotile as a support for the immobilisation of Mycobacterium sp. NRRL B-3805 cells for the bioconversion of β-sitosterol in an organic–aqueous two-liquid phase system

The use of chrysotile, a plentiful and low-cost inorganic material, as an effective immobilisation matrix for mycobacterial cells was evaluated. The side-chain cleavage of β-sitosterol to 4-androstane-3,17-dione (AD), in an organic–aqueous two-liquid phase system, was chosen as model system. Bis(2-ethylhexyl) phthalate (BEHP), a biocompatible organic solvent, was used as organic phase. The biotransformation was carried out with either growing or previously grown cells. Mycobacterial cells were shown to adsorb onto chrysotile in both aqueous buffered system and in organic–aqueous two-liquid phase systems. Cells tend to aggregate among the chrysotile fibres during and after growth. The formed chrysotile/cells complex retained catalytic activity under bioconversion conditions for 1 week, at least. No mass transfer hindrances were observed when the immobilised form of the biocatalyst was used, as compared to the free form. However, among the several approaches to produce the most efficient immobilised biocatalyst, the use of previously grown cells in contact with chrysotile led to the highest product yields as well as highest bioconversion rates. The feasibility of this approach was further evidenced since it allowed the implementation of a continuous mode of operation. A constant product yield of 50% was observed for a 1-week period with this experimental set-up. This work clearly highlights the ability of chrysotile to provide an efficient immobilisation matrix for whole cells to be used as biocatalysts in organic–aqueous two-liquid phase bioconversion systems.