Plenty of extracellular DNA makes your community stronger

Biofilms are formed by aggregates of bacteria and are held together by extracellular polymeric substances (which consists of a mixture of substrates of bacterially produced polymers), one of these constituents is extracellular DNA (eDNA), it's role is not clearly understood. The advantages of biofilms are thought to be: protection against desiccation, mechanical sheer strength and chemical toxins. The composition of EPS is varied and is dependent on the species present in the biofilm and the growth conditions. Generally speaking it mainly consists of three compounds: polysaccharides, proteins and extracellular DNA. Recent research indicated eDNA is frequently a component of many biofilms. Documented species that produce eDNA are: Pseudomonas aeruginosa, Neisseria gonorrhoeae, Staphyloccocus epidermidis, Shewanella sp., Acinetobacter calcoaceticus and Bacillus subtilus. eDNA is present in both natural and synthetic environments and comprises of up to 70% of total DNA pool in a biofilm and typically ranges from 2 mg g^-1 to 300 mg g^-1. It is thought to originate from either cell lysis or be secreted from live cells. The production of eDNA in P.aeruginosa have been linked to quorum sensing. Research indicates eDNA has several roles: a nutrient source during low nutrient conditions, phosphorus cycling in sea sediments, horizontal gene transfer, and structural strength. The abundance and importance of eDNA in bacterial complexes is not well documented, the authors attribute this to a lack of stable methods for quantitative in situ studies. Methods that are used are qPCR, spectrophotometric detection, DAPI, SYTO, DDAO and PI stains. The aim of Dominiek et al.'s research was to further develop in situ techniques for quantitative analysis of eDNA in mixed biofilms and activated sludge flocks. Their technique was simplistic and combined refining staining methods with in situ hybridisation (fish) for the identification of microbes.

Their staining results indicate that most of the eDNA was found around living cells, more specifically there was increased abundance around microcolonies of certain species of bacteria, which suggests there is variation in the production of eDNA among species. This supports the hypothesis that eDNA originates from live cells, however, it is also suggested that it may provide evidence that sub microcolonies are releasing eDNA during lysis. Confocal FISH images of extracellular floc before and after a 60 minute DNase 1 incubation, showed there was a loss in eDNA as a result of its degradation. Their quantitative results indicate eDNA ranges from 50 to 300 mg g^-1. Fish results reveal that some species in particular produce high levels of floc eDNA: Curvibacter, Thauera, Nitrospira, Accamulibacter and Campetibacter. These were found in three Danish waste water treatments. Results also demonstrated that eDNA provided structural integrity for the biofilms.

The main discussion points are: 1. eDNA is an important component of ESP. 2. The different contents of eDNA in activated sludge in different tested treatment plants is most likely due to different microbial populations. 3. Most eDNA was found around living cells, suggesting active production of this polymer. 4. DNase 1 had substantial defloculating effects on the entire biofilm floc and also indicated low levels of eDNA impacted the EPS matrix and had consequences to floc strength. 5. Structural biofilm strength resulted in better performance in pressure dewatering conditions and gravity drainage, finally 6. The species composition influences the properties of the EPS and thus its function.

The methods they developed to quantify the abundance of eDNA proved effective. New evidence was gathered about particular bacteria that are strong floc producers. This research presents further detailed insight into the structure of constituent parts of biofilms. What I found surprising is that DNA holds a key structural role, I would have thought that cheaper to produce macromolecules would have been used. This leads me to postulate that eDNA plays other roles within biofilms e.g. transformation and nutrient cycling and cellular communication.

Dominiak, D. M., Nielsen, J. L., Nielsen, Per H., (2011) Extracellular DNA is abundant and important for microcolony strength in mixed microbial biofilms. Environmental Microbiology 13(3), 710-721