What Is Biofilm?

Water in the liquid phase is like a dessert for micro-organisms, with few nutrients available for growth. In addition, flowing water provides an unstable environment in which to live. Consequently, in an effort to maintain their viability, miroorganisms seek solid surfaces conditioned with nutrients sufficient for growth. When attracted to a surface, micro-organisms deposit, attach and initiate growth. As they grow and multiply, newly formed cells attach to each other as well as the surface, by creating a 'polymer matrix', or slime.

In man-made water systems, this deposition and growth results in a slime layer, or biofilm, on surfaces in contact with water, but particularly at or near heat exchange surfaces where the temperature is right for growth, and where there is a sufficient nutrient source from scale, sediment, corrosion products, and trapped organic and inorganic molecules supplied by the flowing water.

In general terms, the dominant population will consist of aerobic (requiring oxygen), and anaerobic (living without oxygen) bacteria interspersed with algae (if light is present) and fungi which in turn support predatory organisms such as protozoa and metazoa. These predatory organisms feed on the biofilm like cattle grazing a field and ingest bacterial cells for food.

Biofilm form a safe habitat for pathogenic bacteria like Legionella to live and proliferate. A link has been established between amoeba, a predatory protozoa, and high levels of Legionella in a water system. When an amoeba ingests a Legionella bacteria, unlike normal bacterial cells which are digested, the Legionella organism colonises the amoeba and proliferates within it until the expansion in numbers cannot be contained. This results in the amoeba bursting and releasing huge numbers of Legionella in a concentrated pack into the water. This can then be transported to outlets, disseminated in a aerosol, and if inhaled can lead to Legionnaire's Disease.

It therefore follows that the key to maintaining the quality of water being stored and transported through a man-made water system is to control the development of biofilm within that system.