Inhibition by ethanol of the growth of biofilm and dispersed microcosm dental plaques

Inhibition of microcosm plaque biofilm growth by periodic application of ethanol was compared with the minimum inhibitory concentration (MIC) and bactericidal effects of ethanol on liquid cultures of dispersed plaque bacteria. Microcosm plaques were cultured from saliva in a multiplaque 'artificial mouth' and their growth in wet weight measured daily. Nutrient conditions included: a continuous supply of a medium containing 0.25% mucin, and 8-hourly 5% (w/v) sucrose (1.5 ml over 6 min). Plaque biofilm growth was strongly inhibited by exposure to 40% (v/v) ethanol applied in volumes of 3.75 ml over 15 min, six times daily. Application df 1.5 ml over 6 min inhibited much less or not at all. Ethanol concentrations lower than 40% caused less inhibition, with 10% having almost no effect. The pH response to sucrose was unchanged by prior application of 40% ethanol for 30 min. Some evidence was obtained for either bacterial adaptation to ethanol or selection of ethanol-resistant bacteria. The MIC and bactericidal effects of ethanol were assessed by growth of dispersed plaque in liquid culture; the bactericidal effect was measured as the induced delay in growth. The aerobic and anaerobic MIC of ethanol for growth was 10% and 8%; 50% inhibition of growth rate occurred at 3.7% and 2.8%. Ethanol (40%) was bactericidal within 1-2 min, but 10% had almost no effect. It was concluded that, despite the well-known high ethanol sensitivity of dispersed plaque bacteria, prolonged application of ethanol concentrations in the order of 40% are necessary to inhibit growth of plaque biofilms.