Minimum Inhibitory Concentration of Chlorhexidine and Cetylpyridinium Chloride against a Mixture of Two Species of Oral Streptococci

Background and Objectives: Although bacteria in plaques are present as a mixed population comprising various species, mechanisms underlying differences in susceptibility between the mixed population of bacteria and each individual bacterium to antimicrobial agents is yet unknown. In this study, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of chlorhexidine and cetylpyridinium chloride were determined against various streptococci isolated from the human oral cavity. Then, changes in susceptibility of planktonic bacteria to chlorhexidine and cetylpyridinium chloride were investigated by mixing each of the bacteria in different combinations.

Materials and Methods: MIC and MBC were measured by the micro-dilution method according to the standards recommended by Clinical and Laboratory Standards Institute (CLSI).

Results: The MIC and MBC values of cetylpyridinium chloride against each bacterium tended to be high or more than the high susceptibility values for the two mixed bacteria in all combinations. Most of the MIC and MBC values of chlorhexidine against individual bacterium were higher than those against the mixtures of two bacteria. However, in some combinations, susceptibility values for two mixed bacteria were low or lesser than the low values for the individual bacterium.

Conclusion: When two antimicrobials were applied to mixed bacteria, cetylpyridinium chloride was observed to inhibit the growth of all combinations, with higher MIC and MBC values, whereas chlorhexidine was observed to inhibit the growth to varying degrees, with a different MIC and MBC values.