The Role of Chemostats in the Evaluation of Antimicrobial Agents for Use in Dental Products

Abstract

Dental plaque develops naturally on teeth and forms part of the natural defence systems of the mouth by acting as a barrier to colonisation by exogenous species. 10 Plaque preferentially forms at stagnant or retentive sites and, unless removed by diligent oral hygiene, it can predispose a site to either caries or periodontal diseases. In disease, the balance ofthe resident microflora shifts. In caries, there is an increase in acidogenic and aciduric species (e.g. mutans streptococci, lactobacilli) at the expense of many acid-sensitive species (e.g. Streptococcus sanguis, S. oralis ), while in periodontal diseases there is an increase both in plaque mass and in the proportions of several obligately anaerobic and capnophilic species. For many subjects, the customary oral hygiene of tooth brushing is usually insufficient by itself over long periods to provide a level of plaque control compatible with oral health. Consequently, the incorporation of antimicrobial agents into dental products as a means of controlling plaque and disease has been advocated for a number of years.'' There are several important differences in the way antimicrobial agents function in the mouth compared to the treatment of most medical infections. Agents for plaque control are applied topically, and are usually delivered by toothpaste or mouthwash. Inhibitors are present at concentrations greater than minimum inhibitory concentrations (MICs) of target organisms for a relatively short time (perhaps minutes), and function thereafter for much longer periods (hours) at sub-MIC levels. Also, antimicrobial agents in dental products are required to produce clinical benefit without disrupting the natural ecology of the resident microflora. The longterm use of these agents is intended, therefore, to control rather than eliminate plaque, and to preserve the beneficial properties of plaque. 10,11 The conventional methods of evaluating antimicrobial agents for use in dental products still centre around those used in medical microbiology, such as determining the MIC. As discussed above (and elsewhere in this workshop), the MIC is not particularly relevant to the mode of action of antimicrobials in the mouth. It is also difficult to extrapolate from such simple tests to predict the likely behaviour of an inhibitor in vivo , so more stringent methods for the laboratory assessment of potential antimicrobial agents have been sought. Of particular value has been the use of continuous culture techniques to grow oral bacteria for prolonged periods, under defined and relevant conditions, in pure and mixed culture.