Biofilm Self-destruction!

Bacteria within biofilms are distinctly different from their planktonic counterparts; one of the largest differences being the extracellular matrix that they secrete and live within. This matrix is made up of extracellular polymeric substances (EPS) including considerable amounts of DNA which are important for the initial adhesion and formation of the biofilm, as well as structurally. Bacteria are also capable of modifying these structures through the secretion of matrix degrading enzymes which allow the bacteria escape their biofilm and disperse.

This paper looked at the efficacy of a supernatant secreted by a marine isolate of Bacillus licheniformis for degrading biofilm matrices and aiding dispersal. The supernatant, a nuclease called NucB, was only produced by the biofilm form of B. licheniformis, not the planktonic form. It was effective at formation inhibition and dispersal of both Gram positive and negative bacterial biofilms, including those of B. licheniformis, B. subtilis, Escherichia coli, Micrococcus luteus and Pseudomonas sp. In fact, its dispersal-inducing capabilities were rapid with partial dispersal visible at 2.5 minutes and at higher concentrations, it produced total dispersal in 12 minutes.

The effects of NucB were also compared to those of a bovine nuclease (DNaseI) which is already used to disrupt medically important biofilms. The two substances had very different response curves with regard to dispersing biofilms as DNase1 had a shallower curve indicating prolonged efficacy, compared to the steeper curve of NucB which suggests that its effectiveness dropped off rapidly below a certain concentration. However, although DNaseI worked for longer, below 15ng/ml it only produced partial dispersal until 0.7ng/ml where it ceased to be effective. Comparatively, NucB application resulted in full dispersal of biofilms down to 3ng/ml, suggesting that it is more effective at disrupting bacterial DNA and also implying that this substance could be developed into a compound to control biofilms.

Furthermore, the authors infer from their results that this enzyme is not only useful in the dispersal of native biofilms but also in competing with foreign biofilms. By secreting NucB, bacteria can inhibit the growth of other biofilms in a ‘controlled and precise manor’; thus reducing competition for resources. Therefore, if this enzyme could be applied to surfaces somehow, it could inhibit the growth of biofilms and protect structures from damage.

Another potential use for NucB secretion could be to allow bacteria to escape foreign biofilms that may trap them in their matrix. As the DNA in the matrix is ‘sticky’ (central to its role in adhesion) it is also used to trap unwanted bacteria that come into contact with the biofilm, particularly invasive pathogens. By secreting NucB, pathogens can potentially free themselves from these DNA traps and infect other organisms. This would certainly be a negative side-effect of introducing NucB as a biofilm controlling substance and could restrict its use in public areas.

The applications of this papers findings were not really explored by the authors which is surprising given that they compared NucB with an enzyme which is already used. However, overall the paper is well written and not difficult to follow.

A review of: Nijland R, Hall MJ, Burgess JG (2010) Dispersal of Biofilms by Secreted, Matrix Degrading, Bacterial DNase. PLoS ONE 5(12): e15668. doi:10.1371/journal.pone.0015668