Inhibitory Activities of Surface-Associated Bacteria from California and Florida Algae

Marine algal surface-associated bacteria (SAB) inhabit highly competitive and diverse marine environments, where they face challenges such as high salinity, osmotic stress, and UV radiation. In response, these bacteria produce secondary metabolites that may serve as chemical defenses, including potential antibiotics active against human pathogens. To discover novel antibiotic drug leads, algae samples were collected from three California beaches (Stinson Beach, La Jolla, and Santa Cruz) and the Florida Keys. SABs were isolated from plates where algal surface swabs were plated on A1 medium, then cultured in liquid A1 medium (10 g/L starch, 4 g/L yeast, 2 g/L peptone), and cryopreserved. We utilized 3 antimicrobial screening methods—pour-over assay, disk diffusion, and single-dose broth assay— to test the 532 isolates against four human pathogens: Escherichia coli, Bacillus subtilis, Staphylococcus aureus, and Salmonella sp. In the pour-over assay, 11 isolates from an initial pool of 332 California SABs and 17 isolates from approximately 200 Florida SABs, produced zones of inhibition indicative of antibiotic production. The disk diffusion assay also confirmed antibiotic production in 14 samples. These antibiotic-producing isolates were then cultured large scale in 500 mL A1 media for 48 hours, extracted using XAD-16 resin, and then the broth was further extracted using ethyl acetate. Single-dose broth assay was conducted on the fractions from the large-scale extracts. The next steps will be isolation and characterization of the antibiotic secondary metabolites.