Interactions of Mycobacterium fortuitum with solid sterol substrate particles

Abstract Mycobacterium fortuitum NRRL B‐8119 transformes sterols into 9 alpha‐hydroxy‐androsta‐4‐ene‐3,17‐dione (9OH‐AD) at high efficieny. Cells strongly aggregate to the surface of the sterol particles forming stable agglomerates (“substrate immobilized cells”). Substrate uptake and product formation were studied as function of the size of the sterol particles. Using particle sizes comparable with the size of the mycobacterial cells (> 5μm) highest rates were found for both the substrate uptake and the product formation. After mechanical reinforcement of the contact between mycobacterial cells and the sterol substrate by co‐grinding of the components, a significantly increased product formation was observed. Associated cells and cell‐sterol agglomerates were investigated by freeze‐fracture electron microscopy. The micrographs obtained demonstrate that cells of M. fortuitum are growing into the sterol microcrystallite. The uptake of the sterol substrate is assumed to take place via direct contact between cells and the substrate particles. To understand the transport mechanism a model is proposed that includes a flexible multicomponent mesophase (FMCM) which is placed between cells and particles and which mediates the sterol uptake. The putative FMCM is assumed to be composed of mycobacterial glycolipids, extracellular biolipids, synthetical detergents, sterol and water.