Chemical inhibition of bacterial colonization by the red alga Bonnemaisonia hamifera

Attachment and growth are 2 major processes in bacterial colonization of surfaces in the sea. By inhibiting either or both of these processes, marine macroorganisms may defend themselves against bacterial infection and fouling. We tested crude extracts from 5 red seaweed species for their ability to inhibit bacterial growth and attachment. For this we used 11 strains of bacteria, representing 5 different taxonomic groups. The effects on growth and attachment were tested by a standard disc-diffusion assay and by incorporating crude extracts into phytagel blocks that served as a surface for bacterial attachment. Extracts from one of the tested algae, Bonnemaisonia hamifera, were particularly active and inhibited growth of 9 bacteria at concentrations volumetrically equivalent to whole algal tissue, or lower. The other 4 algal extracts had weak growth-inhibiting effects on only a few bacterial strains. None of the algal extracts exhibited broad-spectrum effects against bacterial attachment, but 4 of 5 algal extracts had some strain-specific effects. Surface extracts of B. hamifera tested on bacteria showed that metabolites are naturally present at sufficiently high concentrations in order to inhibit bacterial growth on the surface of the seaweed. In situ quantification of bacteria on B. hamifera also showed that this alga had significantly fewer bacteria on its surface compared to a coexisting alga. These findings suggest that B. hamifera naturally reduces its epibacterial abundance by production of broad-spectrum growth-inhibiting secondary metabolites. This is one of a few examples where ecologically relevant effects of algal metabolites on bacterial colonization have been shown.