Amphiphilic property of chlorhexidine and its toxicity against Streptococcus mutans GS-5

The toxicity of chlorhexidine digluconate against Streptococcus mutans GS-5 was comparatively determined by measuring its bactericidal activity, and its inhibitory effect on microbial dehydrogenases was studied by the resazurin reduction method. Both methods indicated that chlorhexidine within the range 0.75-5.00 mg/l was highly toxic to Streptococcus mutans, probably due to inhibition of dehydrogenase activity, and the extent of toxicity was closely associated with concentration. In an attempt to better understand the relationship between the amphiphilic nature of chlorhexidine and its toxicity against Streptococcus mutans, the effect of solvent polarity on dehydrogenase inhibition was investigated. A decrease in solvent polarity, induced by inclusion of 5% acetone in the reaction mixture, did not enhance the toxicity of chlorhexidine. This implies that the antimicrobial action of chlorhexidine is mainly attributable to its hydrophilicity, and that the nature of the lipophilic groups is only of secondary importance.