Inhibition of bacterial fouling by soft coral natural products

• Surface of the soft corals Sinularia sp.1 and Cladiella sp. had the lowest densities of bacteria.
• Only extracts of Cladiella sp. inhibited growth of biofouling bacteria.
• Antibacterial compounds were hexadecyl palmitate (C32H64O2) and hexadecyl stearate (C34H68O2).
• Hexadecyl palmitate inhibited growth and attachment of marine fouling bacteria.
• This is the first report that long chain carboxylic acid esters inhibit bacterial fouling.

Six common soft coral species (Sarcophyton sp., Sinularia sp.1 and sp.2, Cladiella sp., Scleronephthya sp. and Dendronephthya sp.) from Bandar Al-Khayran (Sultanate of Oman) had significantly lower bacterial density in comparison with surfaces of empty shells. Methanol: chloroform (1:1) extracts of these species were tested against Gram positive (Micrococcus luteus, Staphlococcus aureus, Bacillus subtilis, Bacillus sp.) and Gram negative (Salmonella enterica, Pseudomonas aeruginosa, Escherichia coli, Cytophaga sp., Pseudomonas sp., Shewanella sp.) marine biofouling and pathogenic bacterial strains. All tested extracts had some activity against human pathogens and the highest antimicrobial activity was observed for extracts of Sinularia sp.1 and Cladiella sp. (inhibited 50% and 60% of the strains, respectively). Only ethyl acetate extracts of Cladiella sp. inhibited growth of biofouling bacteria. The active fraction was purified and identified as a mixture of hexadecyl palmitate and hexadecyl stearate. Pure hexadecyl palmitate inhibited growth (Bacillus sp. and Psychrobacter sp.) and attachment (Bacillus sp., Cytophaga sp., Pseudomonas sp., Psychrobacter sp., Shewanella sp.) of the marine biofouling bacteria. The results of this study suggest that soft corals have developed mechanisms to combat microbial infections and inhibit bacterial fouling.