Antibacterial Potential of Secondary Metabolites from Indonesian Marine Bacterial Symbionts

Indonesian marine environments are known to house diverse organisms. However, the potential for bacteria from these environments as a source of antibacterial agents has not been widely studied. This study aims to explore the antibacterial potential of secondary metabolites produced by bacterial symbionts from sponges and corals collected in the Indonesian waters. Extracts of 12 bacterial isolates from sponges or corals were prepared by cultivating the bacteria under a number of different media conditions and using agar well diffusion assays to test for antibacterial activity. In addition, the morphology, physiology, and biochemical characteristics and 16S rRNA sequence of each isolate were used to determine their taxonomic classification. All tested bacterial isolates were able to produce secondary metabolites with various levels of antibacterial activity depending on medium composition and culture conditions. Two of the bacteria (RS3 and RC4) showed strong antibacterial activities against both Gram-negative and Gram-positive bacteria. A number of isolates (RS1, RS3, and RC2) were co-cultured with mycolic acid-containing bacteria, Mycobacterium smegmatis or Rhodococcus sp. However, no improvements in their antibacterial activity were observed. All of the 12 bacteria tested were identified as Streptomyces spp. LC-MS analysis of EtOAc extracts from the most active strains RS3 and RC4 revealed the presence of a number of dactinomycin analogues and potentially new secondary metabolites. Symbiotic Streptomyces spp. from sponges and corals of the Indonesian marine environments have great potential as a source of broad-spectrum antibacterial agents.