Antibacterial Sensitivity Changes in Mixed Species Biofilm of Oral Streptococci against Chlorhexidine and Cetylpyridinium Chloride

Background and Objectives: Although plaques are present in a mixed state of various bacterial species, it is not yet known how antimicrobial efficacy of mixed bacteria against the antimicrobial agent changes compared to susceptibility of individual bacteria to the antimicrobial agent. In this study, antibacterial effects of chlorhexidine and cetylpyridinium chloride were observed in biofilm state after mixing two bacteria to observe the change of susceptibility value when bacteria were mixed with each other.

Materials and Methods: The minimum biofilm inhibitory concentration (MBIC) and the minimum biofilm eradiation concentration (MBEC) were determined to determine the susceptibility of the antimicrobial agent after the formation of the biofilm, and were determined by microtiter plate method according to the previously reported method.

Results: When two bacteria with different susceptibility values were mixed, it was observed that most of the combinations had higher values of susceptibility values of the two bacteria. However, in some bacterial combinations, susceptibility values of the two bacteria revealed various results such as following the low value, lower than the low value, or higher than the high value.

Conclusion: This study has demonstrated that antimicrobial susceptibility to mixed bacteria can change in a variety of ways without simply following high values.