Effects of Extracellular Plaque Components on the Chlorhexidine Sensitivity of Strains of Streptococcus mutans and Human Dental Plaque

Abstract

An in vitro study was undertaken to determine the effects of sucrose-derived extracellular plaque components on the sensitivity of selected oral bacteria to chlorhexidine (CX). Cultures of Streptococcus mutans HS-6, OMZ-176, Ingbritt C, 6715-wtl3, and pooled human plaque were grown in trypticase soy media with or without 1 % sucrose. The sensitivity to CX of bacteria grown in each medium was determined by fixed-time exposure to CX and subsequent measurement of ³H-thymidine uptake. One-hour exposure to CX at concentrations of 10-4 M (0.01% w/v) or greater substantially inhibited subsequent cellular division among all the S. mutans strains and human plaque samples tested. An IC₅₀ (the CX concentration which depressed ³H-thymidine incorporation to 50% of control level) of close to 10^-4 M was noted for S. mutans strains HS- 6, OMZ-176, and 6715-wt13 when grown in the presence of sucrose. The same strains grown in cultures without added sucrose showed about a ten-fold greater sensitivity to CX (IC₅₀ close to 10 ^-5 M). A three-fold difference was noted for S. mutans Ingbritt C. Only a slight increase in the IC₅₀ was noted for the plaque samples cultured in sucrose-containing media, but their threshold for depression of ³H-thymidine uptake by CX was lower than that for the sucrose-free plaque samples. The study showed that extracellular products confer some protection against CX to the bacteria examined, and provided an explanation for the disparity between clinically-recommended concentrations for plaque suppression and data on in vitro susceptibility. Also, when compared with similarly-derived indices of susceptibility of mammalian cells to CX, the data obtained give rise to new possibilities for therapeutic use of CX or other bisguanides in dentistry.