Biocide treatment of biofilms

Biofilms are ubiquitous in nature and microorganisms often exist as members of complex consortia, rather than as pure cultures. Their localised metabolic activity can create diffusion gradients of nutrients, fermentation byproducts and possible associated corrosion products within the biofilms; together with cell lysis, these cause a mosaic of microenvironments which may be totally different to the bathing phase. Such habitats pose a major, and often ignored, constraint on the interpretation of results obtained from laboratory disinfection models which can be physically, environmentally and physiologically inappropriate. For example, the most commonly used model for inactivation of microorganisms by biocides utilises the so-called ‘Chick- Watson law’: this implies that biocide concentration and contact time, the (C × T) factor, are the two key variables determining biocide efficacy. However, applications of the ‘law’ have assumed complete and uniform mixing of microorganisms and biocide, ignoring that diffusion might be rate limiting and that biocide concentration might decrease with time. Recent results suggest that many of the viable bacteria in chlorinated potable water are attached to surfaces and under these circumstances coliforms have withstood at least 12 ppm free residual chlorine. The use and efficacy of alternative biocides such as monochloramine against aquatic biofilms is discussed.