What is a biofilm?

Biofilms are common in the natural world. They are made up of microorganisms and occur primarily on surfaces at boundaries between solid and liquid phases. The vast majority of the earth’s microorganisms (99 %) live in biofilms.1 They are embedded in a layer of mucus which settles on the surfaces like a coating. These communities, which can include bacteria, fungi, protozoa and algae, are very well adapted to their surroundings and are more resistant to environmental conditions than free-floating planktonic cells. This is because of the complexity of the biofilm, the increased cell density, the physiological nature of the cells and the differential gene expression in biofilms. Enteric viruses can also settle in biofilms. The precise interactions between viruses and biofilms are currently being investigated with reference to both physicochemical and biological parameters.² 

The microorganisms organised in these communities reproduce within the biofilm by making use of the metabolic properties and protective mechanisms of the other species. Biofilms form on stones in a river bed but they also form on the inside of water pipes – a fact that plays an important role in drinking water hygiene. In the human body, biofilms can occur on structures such as implants where they are usually associated with health problems. However, they can also perform useful functions, for example in the gastrointestinal tract where they prevent colonisation by pathogens.

How does a biofilm form?

A damp environment provides ideal conditions for the reproduction of microorganisms. Some types of bacteria are able to form a layer of mucus (extracellular polymeric substances, EPSs) with which they stick themselves onto surfaces. This layer forms the matrix and structure of biofilms. The EPSs consist of polysaccharides, numerous proteins, lipids, phospholipids, glycoproteins, glycolipids, often extracellular DNA (e-DNA), lipopolysaccharides (LPS) and up to 97 % water. The growing matrix makes it easier for other microorganisms to settle. As the matrix grows the biofilm spreads laterally and forms three dimensional structures consisting of several layers. By living as a community in this way, the organisms are better able to survive. The extracellular matrix offers benefits including mechanical protection against external attack. In certain areas, such as water pipes, a biofilm can form within just a few days, even when the water is of high quality.  

What effects do biofilms have?

Although there are areas in nature where biofilms are very useful, such as self-cleaning processes in soils and bodies of water or the binding of carbon dioxide, they constitute a major problem in other areas. The microorganisms in biofilms can include such organisms as Legionella bacteria and pseudomonads (e.g. Pseudomonas aeruginosa) which are pathogenic to humans. Pseudomonads can cause various human infections including pneumonia, urinary tract infections and wound infections. Other pathogens often found in biofilms are Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli and Candida albicans.

People come into contact with these pathogenic bacteria via taps, shower heads and waste traps. However, the final rinse in the purification of medical devices such as endoscopes can also allow waterborne pathogens to be deposited on the devices after cleaning.  

Once a biofilm has become established it is almost impossible to remove the film completely. More than 45 % of all hospital infections can be traced back to medical devices that have been contaminated by biofilms.³ 

Contaminated water pipes constitute a health risk

Low flow rates and dead end pipes in the water system favour the formation of biofilm. When the mucous layer reaches a certain thickness, the power of the flowing water causes pieces of film to be dislodged and released into the water system. These then settle at other points in the water system or travel to a water outlet and reach the users.

It is almost impossible to remove a biofilm completely once it has formed because the film protects its microorganisms against chemical and thermal disinfection procedures.

For immunosuppressed patients, in particular, hospitals therefore resort to physical filtration of the water using membrane filters. The hollow fibre membrane technology enables these sterile water filters to hold back all waterborne microbes. That includes Legionella bacteria (e.g. Legionella pneumophila) and pseudomonads.



1 Costerton et al. 1987
3 Bixler & Bhushan, 2012