Why is extracellular DNA contained in biofilms?


Biofilms are microorganisms on surfaces that are widespread in nature - especially on the interfaces between a solid and a liquid phase. Most of the microorganisms on earth (99%) live in biofilms.1 They are embedded in a layer of slime that settles on the surfaces like a coating. These communities of bacteria, fungi, protozoa or algae are very well adapted to their environment and are more resistant to environmental conditions than free-swimming, planktonic cells. The reasons for this are the complexity of the biofilm, the increased cell density, the physiological cell structure and the differential gene expression in biofilms. Enteral viruses can also lodge in biofilms. The exact interactions between viruses and biofilms are currently being investigated using physicochemical and biological parameters

The microorganisms organized in biofilms use metabolic properties of the other species or their protective mechanisms to multiply in biofilms. Biofilms form on stones in a stream bed, but also on the inside of water pipes - a fact that plays a major role in drinking water hygiene. Biofilms can be found in the human body, e.g. on implants, which is usually associated with adverse health effects. But they can also have useful functions, e.g. B. in the gastrointestinal tract, they prevent the settlement of pathogenic germs.

How is biofilm created?

A moist environment offers microorganisms ideal conditions for reproduction. Some types of bacteria are able to form a layer of mucus (extracellular polymeric substances, EPS) that they use to attach to surfaces. This layer forms the matrix and structure of the biofilms. The EPS mainly consist of polysaccharides, a large number of proteins, lipids, phospholipids, glycoproteins, glycolipids, often extracellular DNA (e-DNA), lipopolysaccharides (LPS) and up to 97% of water. The growing matrix makes it easier for other microorganisms to settle. This leads to the lateral expansion of the biofilms and the build-up of three-dimensional structures made up of several layers. Through this community, the organisms are able to survive. The extracellular matrix also offers mechanical protection against external attacks. In certain areas, such as B. water pipes, biofilm can form after a few days, even if the water is of a high quality.

What does biofilm do?

Although there are areas in nature where biofilms are very useful, e.g. B. Self-cleaning processes in soils and waters or the binding of carbon dioxide, they represent a major problem in other areas. The microorganisms in the biofilm that are pathogenic for humans include legionella bacteria or pseudomonads (e.g. Pseudomonas aeruginosa). The latter can cause a wide variety of infections in humans, such as B. cause pneumonia, urinary tract infections or wound infections. Other pathogens that are often found in biofilms are Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli, Candida albicans.

Humans come into contact with these pathogenic bacteria via taps, shower heads and siphons. But also with the so-called final rinse in the cleaning process of medical devices such. B. Endoscopes, water germs can be entered in the devices after cleaning.

Once a biofilm has established itself, it is almost impossible to completely remove it. Medical equipment contaminated with biofilm is responsible for more than 45% of all hospital infections.³

Contaminated pipelines pose a health risk

A low flow rate or dead strands favor the formation of biofilm. In the water pipe systems parts - when the slime layer has reached a certain thickness - detach themselves through the force of the flowing water and settle again in other places in the pipe system or reach the user of a water point.

It is almost impossible to completely remove a biofilm once it has formed, because it protects the microorganisms it contains from chemical and thermal disinfection measures.

Hospitals therefore resort to manual filtering of the water with the help of membrane filters, especially with immunosuppressed patients. Due to their hollow fiber membrane technology, these sterile water filters hold back all water-bound germs, including legionella (e.g. Legionella pneumophila) and pseudomonads.



1Costerton et al. 1987


3Bixler & Bhushan, 2012