Influence of Sub-Lethal and Lethal Concentrations of Chlorhexidine on Morphology and Glucosyltransferase Genes Expression in Streptococcus mutans UA159

Abstract

Chlorhexidine (CHX) is regarded as one of the most successful antiplaque agents in controlling the formation of dental biofilm. Nevertheless, molecular mechanisms of their effects in Streptococcus mutans are largely unknown. In this work, the effects of sub-lethal and lethal concentrations of chlorhexidine (CHX) on planktonic or biofilm-organized Streptococcus mutans cells were investigated in dose- and time-dependent manner. The Minimum Inhibitory Concentration (MIC) and the Minimum Bactericidal Concentration (MBC) for planktonic cells and biofilm conditions were determined by standard methods. Quantitative PCR (qPCR) was used to quantify the relative levels of glucosyltransferase B (gtfB), gtfC and gtfD transcription of S. mutans in the presence of CHX. The CHX activity in the initial biofilm structure and morphological alterations in planktonic cells were examined by Scanning Electron Microscopy (SEM). The results indicate that CHX increased expression of gtfC and gtfD in planktonic S. mutans cells and CHX reduced the expression of gtfB, gtfC, and gtfD in biofilms. High concentrations of CHX resulted in several wilted S. mutans planktonic cells with spilled intracellular material, while decreased cells’ chain length and matrix was found when the initial biofilm was exposed to increasing concentrations of CHX. CHX’s effects against bacteria depend on the type of growth organization and the concentration and time of exposure to the drug. At sub-lethal concentrations, CHX affects the expression of glucosyltransferases, which may have anticariogenic effect.