Culture medium modelisation for optimization of anti methycillin resistant Staphylococcus aureus metabolites by a coal mining soil derived Streptomyces rochei CMB47 strain

The urgent need for the development of new antibacterial drugs arises from the global increase of antibiotic resistance. In addressing this challenge, Streptomyces rochei CMB47, isolated from a Saharan soil of a coal mine provided an ethyl acetate extract which exhibited a minimum inhibitory concentration of <0.439 µg/mL against MRSA, resulting therefore a promising source of bioactive metabolites.

Medium formulation and optimization is essential for the success of an industrial production by fermentation, as it directly affects time and costs of the bioactive products.

A statistical medium optimization experimental design employing a response surface methodology (RSM) based on a second-order rotatable central composite design (RCCD) was employed to optimize the fermentation process, in favor of an enhanced production of the metabolites responsible of the bioactivity. Optimal conditions for starch and NaNO₃ concentrations, incubation time, and initial pH were determined, resulting in an inhibition zone diameter of 20 mm, closely matching the experimental value after model validation. Bioassay-guided fractionation of the crude extract yielded the most active fractions, afterwards subjected to high-performance liquid chromatography equipped with a photodiode array detector and coupled online with electrospray mass spectrometry (HPLC-DAD/ESI-MS). These results provided preliminary insights into the molecular structures of the metabolites.