Antimicrobial activity observed among cultured marine epiphytic bacteria reflects their potential as a source of new drugs

The surfaces of marine eukaryotes provide a unique habitat for colonizing microorganisms where competition between members of these communities and chemically mediated interactions with their host are thought to influence both microbial diversity and function. For example, it is believed that marine eukaryotes may use their surface-associated bacteria to produce bioactive compounds in defence against competition and to protect the host against further colonization. With the increasing need for novel drug discovery, marine epibiotic bacteria may thus represent a largely underexplored source of new antimicrobial compounds. In the current study, 325 bacterial isolates were obtained from the surfaces of marine algae Delisea pulchra and Ulva australis. Thirty-nine showed to have antimicrobial activity and were identified via 16S rRNA gene sequencing. The majority of those isolates belonged to Alpha- and Gammaproteobacteria. Interestingly, the most commonly isolated bacterial strain, Microbulbifer sp., from the surface of D. pulchra has previously been described as an ecologically significant epibiont of different marine eukaryotes. Other antimicrobial isolates obtained in this study belonged to the phyla Actinobacteria, Firmicutes and Bacteroidetes. Phylogenetically, little overlap was observed among the bacteria obtained from surfaces of D. pulchra and U. australis. The high abundance of cultured isolates that produce antimicrobials suggest that culturing remains a powerful resource for exploring novel bioactives of bacterial origin.