Statistical Medium Optimization for the Production of Anti-Methicillin-Resistant Staphylococcus aureus Metabolites from a Coal-Mining-Soil-Derived Streptomyces rochei CMB47

The development of novel antibacterial drugs needs urgent action due to the global emergence of antibiotic resistance. In this challenge, actinobacterial strains from arid ecosystems are proving to be promising sources of new bioactive metabolites. The identified Streptomyces rochei strain CMB47, isolated from coal mine Saharan soil, provided an ethyl acetate extract which tested against a series of pathogens. It displayed a minimum inhibitory concentration of <0.439 µg/mL against MRSA. A statistical experimental design using a response surface methodology (RSM) based on the second-order rotatable central composite design (RCCD) was planned to develop an efficient fermentation process able to improve the bioactive metabolite production. The optimal conditions were determined for starch and NaNO3 concentrations, incubation time and the initial pH value, reaching the inhibition zone diameter of 20 mm, close to the experimental value, after validation of the model. A bioassay-guided fractionation of the crude extract provided the most active fractions, which were analyzed by HPLC equipped with a photodiode array detector and coupled online with an electrospray mass spectrometer (HPLC-DAD/ESI-MS), obtaining preliminary indications on the molecular structures of the metabolites. These results support the potential interest in further investigations into the purification and full characterization of the metabolites responsible for the biological activity observed so far.