Effects of low-concentrated chlorhexidine on growth of Streptococcus sobrinus and primary human gingival fibroblasts

The aim of this study was to investigate the in vitro effects of low concentrations of chlorhexidine (CHX) on the proliferation of Streptococcus sobrinus (ATCC 33478) and primary human gingival fibroblasts (HGF). Liquid cultures of bacteria or human gingival fibroblasts were exposed to CHX concentrations ranging from 0.07 to 40 microM in microtiter plates at 37 degrees C for 24 h. Bacteria or cells grown without CHX served as controls. The effects of CHX were determined either by measurements of the optical density (OD) of Bacterial cultures or by evaluation of cell growth with the DNA-intercalating fluorescent stain H33342 in comparison to untreated controls. Results were evaluated calculating means and standard deviations. Data were statistically analyzed by an ANOVA using Post Hoc tests ( p<0.005). No growth inhibition of S. sobrinus was found at concentrations between 0.07 and 0.15 microM CHX, whereas 0.3 microM CHX led to an elongated (2 h more) lag phase and 0.6 microM CHX induced a lag phase of 4 h more and a minor inclination of the curve in the log phase. Concentrations of CHX>/=1.25 microM completely inhibited growth of S. sobrinus. On the contrary, CHX showed no significant effect on growth of HGF at concentrations </=5 microM. A slight growth inhibition was only observed at a concentration of 5 microM. CHX-concentrations of 10 and 20 microM reduced cell growth to 88 or 75% of control assays. Data analysis showed that overall treatment effects were highly significant ( p<0.005). Our data reveal that chlorhexidine inhibits proliferation of S. sobrinus even at very low concentrations while concentrations of CHX</=5 microM are not cytotoxic to human gingival fibroblasts.