Discovery of antimicrobial activity in chemical extracts derived from unexplored algal-bacterial culture systems and isolates

Global health is affected by viral, bacterial, and fungal infections that cause chronic and often fatal diseases. Identifying novel antimicrobials through innovative methods that are active against human pathogens will create a new, necessary pipeline for chemical discovery and therapeutic development. Our goal was to determine whether algal production systems represent fertile ground for discovery of antibiotics and antifungals. To this end, we collected high-biomass algal-bacterial samples from outdoor mass cultivation systems, 18-L outdoor algal open cultures mesocosms, and non-axenic laboratory samples. We also cultivated 33 marine bacterial isolates for chemical extraction. Ultimately, we filtered, concentrated, extracted, and screened 77 chemically-complex mixtures using a conventional agar-based microbial growth inhibition assay against three microbes: Escherichia coli, Bacillus subtilis, and Candida albicans. We discovered that 23 of our chemical extracts (almost one-third of the chemical samples tested) exhibited some degree of growth inhibition toward B. subtilis and/or C. albicans. Our work here demonstrates the feasibility and potential of isolating bioactive natural products from high-biomass algal-bacterial samples from algal mass cultivation systems.