Susceptibility of planktonic versus attached Streptococcus sanguinis cells to chlorhexidine

The effect of chlorhexidine (CHX) on the viability of Streptococcus sanguinis was investigated in a preclinical biofilm model separately on cells in the planktonic or attached life form. Saliva-coated human enamel and glass slides were exposed to the streptococci suspended in sterile saliva for 30 min and 60 min in the flow chamber system. The CHX exposition was performed in two parts: pretreatment of the planktonic bacteria before their attachment to enamel or glass, and treatment of bacteria already attached to enamel. The susceptibility measured by vitality percentages was determined by fluorescence microscopy using vital/dead cells. After CHX pretreatment of planktonic cells, the mean values of the vitality percentages after adhesion were 14-18% (enamel) and 24-25% (glass). In contrast, the mean vitality percentages of untreated attached streptococci reached 70-75% (enamel) and 68% (glass). The vitality percentages of CHX-exposed bacteria dropped markedly to 2-5%, whereas those of untreated attached cells remained at 65-66%. The exposure of initially attached streptococci to CHX resulted in greater reduction of bacterial viability than with the planktonic counterparts. This preclinical biofilm model allows the investigation of various bacterial life forms and can furthermore be used to select efficient antiplaque therapeutics which might be beneficial for clinical plaque control.