Chemical Warfare in Biofilms

Biofilms are a common mode of life for bacteria in many aquatic ecosystems. One of several suggested reasons for this is the protection that they have been found to offer organisms against predation. A major source of mortality for both planktonic and biofilm bacteria is predation by phagotrophic protists, which rapidly locate and graze aggregations of these organisms causing graze-resistant strains to evolve. In biofilms however these resistant strains do not seem to appear, suggesting chemical defence may play an important role against predation at high cell density. High planktonic populations have been previously found to rely more heavily on cell morphology and escape rather than chemical methods. This study compared the presence of chemical defences in marine biofilms vs plankton populations and found that chemical defences were far more prevalent in biofilm communities. The authors then went on to investigate the nature of one such chemical defence, which is effective against protozoan predators. The potential advantages for bacteria growing as biofilms were tested by contrasting the occurrence of chemical defence in biofilm and plankton populations.

Bacterial communities from the marine macroalga Ulva australis were used in this investigation due to their associations with both planktonic and biofilm habitats. They were isolated for 24 hours before planktonic bacteria were separated their biofilm counterparts. Niche separation necessitated the use of two different protozoan grazing species, as there is not a single species capable of utilizing both biofilm and planktonic bacteria. C.roenbergensis was used in the planktonic communities, while R. nasuta was added to the biofilm bacteria. A significant reduction in planktonic cell numbers was recorded in isolates where predatory protazoa (C. roenbergenis) were present, which contrasted with the findings from biofilm isolates showing high levels of resistance to R. nasuta.

The findings from this investigation provide further evidence for previous research that suggests biofilms provide a refuge from protozoan grazing. Previous work by the authors of this paper proposed that biofilm structure may play a part in the prevention of predation by protozoa. These ideas have been expanded upon here with the suggestion that chemical defence may also play a significant role. Analysis showed that the chemical violacein was released by the bacteria within the biofilm in much higher quantities than in the planktonic communities. Further tests showed that this chemical was toxic to the grazing protozoa, providing further evidence for the idea of chemical defence within marine biofilms. As a note of interest, violacein has been proposed as a possible anti-cancer treatment as it could induce apoptosis in mammalian cell lines.

A review of Matz C., Webb J.S., Schupp P.J., Phang S.Y., Penesyan A., Egan S., Steinberg P. and Kjelleberg S. (2008) Marine Biofilm Bacteria Evade Eukaryotic Predation by Targeted Chemical Defence, Plos, 7, e2744.