Formation of microbial biofilm in hygienic situations: a problem of control

The formation of microbial biofilm presents a challenge to the establishment and maintenance of hygienic conditions in public health, the home and in industry. The use of biocides is central to the hygienic control of microbial growth. Such agents, optimised for their activity against suspended populations of cells, are spectacular in their failure to control adherent biofilm communities. To date there have been limited developments to remedy the situation. Recalcitrance of biofilm communities to anti-microbial treatments has been attributed to organisation of cells within an exopolymeric matrix. This, together with the close proximity of cells, causes reaction-diffusion limitation of the access of agent from its point of application to the deeper lying cells. These cells out-survive those on the surface of the biofilm and, if the bulk of the treatment agent is depleted or the exposure transient, will multiply and divide rapidly. Nutrients also will become depleted within the core of the developing community. This leads to the establishment of spatial gradients of growth rate and redox within the community structure. Different growth-limiting nutrients will also prevail at different points in the biofilm. These conditions provide for a plethora of phenotypes within the biofilm, each reflecting the physico-chemical micro-environment of individual cells and their proximity to neighbours. Faster-growing, more susceptible cells will generally lie on the periphery of the biofilm with slow-growing recalcitrant ones being more deeply placed. The more resistant phenotypes will survive the remainder. In both instances, at the fringes of action, selection pressures will enrich the populations with the least susceptible genotype or phenotype. Repeated chronic exposure to sub-lethal treatments might then select for a resistant population that share this resistance with third party agents. Whilst neither mechanism can provide a complete explanation of recalcitrance, together they will delay eradication of the treated population and allow other selection and regulation events to occur. (C) 2003 Elsevier Science Ltd. All rights reserved.