Comparison of inhibitory dilutions of a thymol-based mouthwash (Orion®) with chlorhexidine on Streptococcus mutans and Streptococcus sanguis

Applying the suitable mouthwash can kill bacteria or reduce the bacterial growth; in the same context, diluting mouthwashes can decrease their probable side effects. In this study, the maximum inhibitory concentrations of two common mouthwashes were evaluated. In this in vitro study, agar dilution and disk diffusion methods were used. Sixty-nine bacterial plates were cultured to evaluate the effects of thymol and chlorhexidine mouthwashes on Streptococcus mutans and Streptococcus sanguis. The results of agar dilution method were recorded as "growth' and "no growth", and the results of disk diffusion were recorded as means of growth inhibitory halos. Data was analyzed by Kruskal-Wallis test using SPSS software. (α = 0.05). Growth inhibitory halos for different concentrations of the two mouthwashes decreased with an increase in dilution rate. In agar dilution method, dilutions less than 80% of the initial concentration for thymol-based mouthwash and less than 70% for chlorhexidine could not inhibit bacterial growth. Growth inhibition halos with 100% thymol-based mouthwash (Orion®) were 10.15 ± 0.1 mm for Steptococcus mutans and 9.2 7 ± 0.16 mm for Streptococcus sanguis. Mean rank of the two mouthwashes was statistically significant for the two bacterial strains under study (p value < 0.001). Both mouthwashes were effective on the bacteria under study; however, this efficacy was higher for Streptococcus sanguis. Chlorhexidine mouthwash was more potent to inhibit bacterial growth than thymol-based mouthwash if diluted.