Breaking Biofilms with DNA

I've written about biofilms a couple of times before but it's an interesting enough topic to keep returning to. As a brief summery, biofilms are large collected colonies of bacteria, often surrounded by a sticky mesh of glycoproteins. They are ultra-annoying in the case of infectious bacteria as the bacteria deep in the depths of the biofilm will not be exposed to any antibiotics, the layers of glycoprotein and surrounding bacteria will protect them.

Although living within a biofilm contains significant advantages (protection, good living conditions, etc) there are also times when the bacteria will want to swim away, in order to disperse and form new colonies. The bacteria C. crescentus has an interesting way of doing this, each round of cell division produces two cells: a moving 'swarmer' cell and a non-motile 'stalked' cell which attaches to the biofilm, or any other surface. If conditions are right, the swarmer cells swarmer cells will eventually turn into stalked cells; loosing their flagellum (which are used to swim), retracting their pili, and growing a membranous 'stalk' to attach it to surfaces or surrounding bacteria. This is shown below:

Lifecycle of Caulobacter crescentus - image from reference 1

Exactly what it was that turned the motile cells into stable ones and maintained the biofilm was not well understood. Recent research found, rather excitingly, that one factor that could lead to the maintenance of swarmer cells, and the breaking up of biofilms, was extracellular DNA (eDNA) - i.e genetic material that had escaped from cells and was floating around the biofilm. Adding eDNA to C. crescentus biofilms lead to biofilm dispersal, an affect that was reversed by adding DNase enzymes that broke down the DNA.

Why is eDNA such an important signal? Because it's one of the most common products produced from dying cells within a biofilm. Once bacteria in a biofilm die, their cellular integrity breaks down, their insides become their outsides and their genetic material spills out into the surrounding area. This can then act as a powerful signal for surrounding cells, and if the cells around you are dying then where you currently are is clearly not a good place to be. This is also useful when areas of biofilm start to get saturated with too many bacteria, just a few dying off will clear the way for new swarmers to leave and maybe set up colonies elsewhere.

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Berne C, Kysela DT, & Brun YV (2010). A bacterial extracellular DNA inhibits settling of motile progeny cells within a biofilm. Molecular microbiology PMID: 20598083

Jermy A (2010). eDNA limits biofilm attachment. Nature reviews. Microbiology, 8 (9) PMID: 20737663

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