IDENTIFICATION OF PROTEIN SYNTHESIS INHIBITORS TARGETING MULTI-DRUG RESISTANT Staphylococcus aureus

Most of the known antibiotics still in use nowadays have been discovered in the 1940s–1960s by extensive screening of soil microorganisms. However, the emergence and spread of antibacterial resistance, in bacteria causing hospital- and community-acquired infections, is dwindling the effectiveness of antibiotics and since that “golden age” few new classes of antimicrobial compounds have been successfully brought to market. Over the last 20 years, neither the search for new antibiotic targets based on genomic and metagenomic approaches, nor the high-throughput screening of synthetic chemical libraries have yielded new and effective antibacterial agents. Natural products, mainly derived from prokaryotic and eukaryotic microorganisms are still the major source of new drug molecules. In this work, we present the results of the screening of a culture collection of microorganisms, searching for producers of new secondary metabolites with antibiotic activity against multi-drug resistant Staphylococcus aureus (MRSA). A preliminary test based on replica plating technique allowed us to select a pool of 45 microorganisms displaying in vivo a wide range of growth inhibition activities toward the reference strain S. aureus ATCC25923. This pool was further restricted to a subset of 11 microorganisms active to different extents against 8 MRSA strains isolated from clinical settings. For the purification of secondary metabolites of the 11 candidate strains, an ‘activity-guided purification approach’ was used, in which every step requires evaluation of antimicrobial activity before the next step is carried out. The supernatant obtained from the cell cultures was fractionated through solid phase extraction (SPE) and the bioactive metabolites were further purified by HPLC. Secondary metabolites isolated from three microorganisms, belonging to the genera Paenibacillus and Bacillus, were found to be targeted inhibitors of the translation apparatus. In fact, the purified molecules, when tested in an in-vitro protein synthesis assay based on model mRNA, showed a marked ability to interfere with incorporation of the radioactively labelled amino acid precursor. The future steps of this project will focus on the evaluation of chemical novelty of the compounds, through NMR spectroscopy and mass spectrometry. The final goal is to find and characterize new compounds that could revitalize the stagnant antibiotic production pipeline and reduce the spread of multidrug-resistant pathogens.