d-leucine enhances antibiofilm activity of chlorhexidine against caries-causing Streptococcus mutans biofilm

Chlorhexidine (CHX) mouthwash is widely used clinically to remove the established Streptococcus mutans biofilm and prevent caries. However, the long-term use with high concentration of CHX leads to the staining of tooth and mucous membrane. In this study, d -leucine ( d -leu) was used as an antimicrobial enhancer for CHX against mature S. mutans biofilm. The biocidal efficacy was evaluated by means of quantitative antibacterial test and biofilm observation. The results indicated that 100 ppm d -leu alone was ineffective to remove the S. mutans biofilm. Moreover, the biocidal effect of 1 ppm CHX was also inadequate to eradicate the biofilm. When 100 ppm d -leu was mixed with 1 ppm CHX, most of the mature S. mutans biofilm on 316L stainless steel coupon surfaces were successfully removed, and the removal efficacy was equivalent to that of 10 ppm CHX alone. The results confirmed that d -leu significantly enhanced the antibiofilm effect of CHX, dissembling the tenacious biofilm into easily killed planktonic counterparts. d -leu represents a novel strategy to reduce the dosage of CHX with a guaranteed effect for caries prevention. • d -leucine disperses the mature biofilm and enhances the biocidal effect of chlorhexidine. • 1 ppm chlorhexidine and 100 ppm d-leucine achieve successful removal of mature biofilm. • d -leucine alone shows no biocidal effect against S. mutans. • d -leucine exhibits no apparent cytotoxicity.