Susceptibility to chlorhexidine of plaque streptococci after two years oral chlorhexidine hygiene

The purpose of the present study was to examine if streptococci, isolated from human dental plaque in experimental subjects having practiced 2 years daily oral chlorhexidine hygiene, showed any difference in susceptibility to chlorhexidine compared to plaque streptococci isolated from a control group. The minimal inhibitory concentrations (MIC) of chlorhexidine were determined using the broth dilution test tube method. From an 18 h culture of each strain in brain heart infusion, 0.2 ml were inoculated into 5 ml brain heart infusion with added chlorhexidine to final concentrations of 0, 0.5, 1, 2, 4, 8, 16, 32, and 64 μg per ml. Three months later identical tests were performed. In both tests strains of Streptococcus mutans, Streptococcus milleri and Streptococcus salivarius were found to be the most susceptible to chlorhexidine, showing MIC around 4 μg chlorhexidine per ml. Strains resembling S. milleri appeared slightly less susceptible than strains identified as S. milleri . The majority of the S. sanguis strains had MIC ranging from 8 to 16 μg per ml. This species was slightly more susceptible than a group of streptococci intermediary to S. sanguis and S. mitior , while the susceptibility of the S. mitior strains ranged from 4 to more than 64 μg per ml. A difference in susceptibility to chlorhexidine between strains isolated from the chlorhexidine group and the control group was only observed for S. sanguis . Strains of this species isolated from the chlorhexidine group showed higher MIC than did strains isolated from the control group.