The attachment of bacteria to a surface and the development of biofilm can be viewed as a survival mechanism. Bacteria benefit from capturing nutrients from the water and developing protection against disinfectants. Potable water, especially high purity water systems, are nutrient-limited environments, but even nutrient concentrations too low to measure are sufficient for microbial growth and reproduction.
How does life in a biofilm help bacteria acquire nutrients? Trace organics will concentrate on surfaces; extracellular polymers will further concentrate trace nutrients from the bulk water; secondary colonisers use the waste products from their neighbours; by pooling their biochemical resources, several species of bacteria, each armed with different enzymes, can break down food supplies that no single species could digest alone.
Biofilm bacteria may be 150-3000 times more resistant to biocides than free-floating bacteria. In order to destroy the cell responsible for forming the biofilm, the disinfectant must first react with the surrounding polysaccharide network. The cells themselves are not actually more resistant, rather they have surrounded themselves with a protective shield. The disinfectant’s oxidising power can be used up before it reaches the cell.
In fact, biofilm bacteria often produce more exopolymers after biocide treatment to further protect themselves. It is important therefore to choose the correct disinfectant and to apply it in the correct manner. A constant application of disinfectant at low strength may actually encourage biofilm growth. Disinfectants based on hydrogen peroxide synergised with silver ions are effective against biofilm. With synergised hydrogen peroxide, the active oxygen arising from the peroxide destroys the biofilm, thus enabling the trace silver components to destroy the bacteria or viruses unhindered.
Biofilm build-up in drains within the bottling plant can cause bad odours and requires a flush through with peroxide to destroy the biofilm and kill odour-causing bacteria. The breakdown products from the peroxide are harmless and can actually benefit surface water by oxygenating the effluent.
Certain other disinfectants are also very effective against biofilm, namely peracetic acid and chlorine dioxide. Peracetic acid works well even at low temperatures, e.g., around 5 degrees C. Chlorine dioxide is highly effective and is recommended for CIP (Cleaning in Place) disinfection programmes.