A mixed-culture chemostat system, composed of nine bacterial species representative of plaque in health and disease, has been assessed as an improved laboratory method of evaluating the likely in vivo effects of antimicrobial agents used in dentistry. The advantages of the system include reproducibility, the long-term stable cultivation of bacteria under controllable conditions, and repeated sampling, for bacteriological and biochemical studies, without disrupting the stability of the community. The effects of (i) the continuous provision of chlorhexidine (CHX) and (ii) three pulses of CHX (final concentration in both experiments = 0.24 mmol/L) on the composition of the chemostat communities were monitored. Only L. casei survived the continuous provision of CHX; the other bacteria were killed and were lost at different rates which generally corresponded to their known sensitivities to CHX. After each CHX pulse, the numbers of bacteria fell markedly. Again, L. casei was least affected, while A. viscosus, B. intermedius, and F. nucleatum were temporarily undetectable but returned to their original levels within 2-4 generation times. Counts of S. mutans were affected more by CHX than those of S. sanguis or S. mitior. The effect of successive pulses of CHX on the viability of some bacteria and on acid production (as measured by pH-fall experiments) decreased, suggesting that adaptation to CHX had occurred. The fact that the in vitro observations paralleled previous clinical findings suggests that the mixed-culture system could be used as a predictive model of the probable effect on the oral flora of new anti-microbial agents prior to expensive trials in animals or human volunteers.
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