Biofilms: there's safety in numbers

A sessile lifestyle can leave organisms vulnerable to predation. To counteract this, many organisms produce secondary metabolites which are deterrent or toxic to consumers – rendering them defensive. Eukaryotes such as corals, sponges and plants commonly produce these defensive chemical compounds, however their production in bacteria is understudied. Bacteria commonly reside as sessile biofilm communities, analogous to sessile macro organisms, and therefore it is likely that chemical compounds are a primary defense strategy against protozoa, a common predator of both biofilm and planktonic bacteria. This study aimed to compare the presence and efficacy of these defenses in biofilm bacteria with that of their planktonic counterparts and went on to investigate the adaptive advantage of biofilm living.

Biofilm colonies on the surface of macro algae were examined and found to hold considerable diversity. Of this, 30 bacterial strains were grown planktonically and as a biofilm and tested for defense against predation.

When living planktonically, all 30 strains were vulnerable as protozoa cell density increased 180-fold on average and all prey bacteria densities were reduced significantly. However, when grown as a biofilm, 17 strains caused a significant reduction in protozoan numbers, indicating resistance or toxicity. Only 11 strains showed reduced cell density and only 8 of these supported protozoan growth. Most interestingly, 19 strains which showed resistance or toxicity to protozoa when grown in a biofilm showed considerable susceptibility when grown planktonically, highlighting the differences between biofilm and planktonic ecotypes.

The author’s previous studies showed that the decrease in protozoan numbers was not due to starvation or the increased difficulty in accessing prey due to biofilm EPS structures, but was most likely due to chemical interference. In order to test this theory the 2 biofilm strains most effective at reducing predator numbers were examined to identify the defensive chemical compound.

Violacein was found to be the putative compound and was also known to be produced by 5 other bacterial species – all of which have a niche preference for sessile communities such as biofilms. Furthermore, when these 5 species were grown in both conditions, all showed significantly higher violacein production rates as biofilms than as planktonic cells. Violacein appears to be a potent compound and affords massive advantages to bacteria that live in biofilms. Firstly, its biosynthesis is likely to be cell density regulated, suggesting that its production is adaptive to colonies but perhaps not to individual cells. Secondly, the digestion of a single cell containing violacein can cause lysis of the predator cell within 1 hour; so although one bacterium dies, it increases the survival of the colony. Lastly, this study shows that low level release of violacein induces an apoptosis-like cell death mechanism, reducing predator cell densities when only a few biofilm cells are consumed. Therefore, biofilm community living can be very adaptive for bacteria and this study comprehensively investigated each aspect of this relative to predation. The findings may have negative implications on biological control of biofilms as they are found to be better equipped against predators than originally thought, however, our improved knowledge of violacein may lead to novel anti-cancer drugs thanks to its autolysis effects.