The extraction and analysis of antimicrobial secondary metabolites produced by cave Streptomyces S1, S4, and PM58B

Antibiotic resistance is a growing problem as microorganisms exhibiting resistance to antibiotics of last resort have been reported in hospitals around the world. To respond to this problem, it is necessary to develop and discover novel antibacterial compounds. A rich reservoir of antibacterial compounds are microbial secondary metabolites, a structurally diverse group of molecules produced by microbes in response to changing environmental conditions providing some advantage to their producers. Particularly prolific producers of these compounds are bacteria of the group Streptomyces, these being responsible for a large portion of the naturally sourced antibacterial compounds used today. In order to access novel and untapped chemical diversity, the investigation of unexplored biological niches has become more prevalent, with the study of cave dwelling microorganisms producing promising leads. This study examines the antimicrobial secondary metabolites produced by three cave Streptomyces strains S1, S4, and PM58b for their previously described activity against target multi-drug resistant Escherichia coli and methicillin resistant Staphylococcus aureus with the goal of determining their molecular characteristics. To achieve this, three strains of Streptomyces were grown for periods of 10 to 30 days to induce the production of secondary metabolites. Bioactivity was confirmed in S1 and S4 by an agar-plug assay and was observed as a pigmented ring of inhibition after an extended period of incubation. Streptomyces strain S1 fermentation broth was extracted and analyzed by matrix assisted laser desorption ionization mass spectrometry for the preliminary identification of the antimicrobial compound present. Antimicrobial activity could not be isolated in a cell free environment from S1 fermentation broths; instead, the antimicrobials were produced on the assay plate. Differences were observed between the fermentation broth and sterile media mass spectra; however, a molecular mass could not be assigned to the putative antimicrobial compound.