MODELING BIOCIDE ACTION AGAINST BIOFILMS

A phenomenological model of biocide action against microbial biofilms was derived. Processes incorporated in the model include bulk flow in and out of a well-mixed reactor, transport of dissolved species into t he biofilm, substrate consumption by bacterial metabolism, bacterial g rowth, advection of cell mass within the biofilm, cell detachment from the biofilm, cell death, and biocide concentration-dependent disinfec tion. Simulations were performed to analyze the general behavior of th e model and to perform preliminary sensitivity analysis to identify ke y input parameters. The model captured several general features of ant imicrobial agent action against biofilms that have been observed widel y by experimenters and practitioners. These included (1) rapid disinfe ction followed by biofilm regrowth, (2) slower detachment than disinfe ction, and (3) reduced susceptibility of microorganisms in biofilms. T he results support the plausibility of a mechanism of biofilm resistan ce in which the biocide is neutralized by reaction with biofilm consti tuents, leading to a reduction in the bulk biocide concentration and, more significantly, biocide concentration gradients within the biofilm . Sensitivity experiments and analyses identified which input paramete rs influence key response variables. Each of three response variables was sensitive to each of the five input parameters, but they were most sensitive to the initial biofilm thickness and next most sensitive to the biocide disinfection rate coefficient. Statistical regression mod eling produced simple equations for approximating the response variabl es for situations within the range of conditions covered by the sensit ivity experiment. The model should be useful as a tool for studying al ternative biocide control strategies. For example, the simulations sug gested that a good; interval between pulses of biocide is the time to minimum thickness. (C) 1996 John Wiley & Sons, Inc.