Phenotypic Adaptation of Oral Bacterial Isolates Toward Chlorhexidine-Digluconate and Cetylpyridinium-Chloride
Objectives: Resistances toward antibiotics have become a major concern for public healthcare. In contrast, studies on development of resistances toward the antiseptics chlorhexidine digluconate (CHX) or cetylpyridinium chloride (CPC) in oral bacteria are scarce, although these antiseptics have been widely used in dentistry for decades. Therefore, the aim of the present study was to investigate phenotypic adaptation of 178 clinical oral bacterial isolates upon repeated exposure to CHX or CPC.
Methods: 113 oral Streptococcus-isolates (14 species), 19 oral Actinomyces-isolates (3 species) and 20 oral Rothia-isolates (14 species) were cultured in Brain-Heart-Infusion-broth, and 26 oral Veillonella-isolates (3 species) in Schaedler-broth, and the minimum inhibitory concentrations (MICs) were determined for CHX or CPC. Bacteria from the sub-MIC population were transferred to the next-day-cycle and exposed to CHX or CPC again. This procedure was repeated for 9 times (passages 1-10; 6 biological replicates each). Isolates showing an 8-fold or higher MIC-increase at passage 10 compared to the MIC at passage 1 were regrown in antiseptic-free nutrient broth for 1, 2 and 3 days and re-evaluated in order to assess the stability of the phenotypic adaptation.
Results: In the CHX group, 23 Streptococcus-isolates and two Actinomyces-isolates showed at least four-fold MIC-increase, and five Streptococcus isolates showed an eight-fold MIC-increase as compared to the MIC of the respective parental strain. In CPC group, six Streptococcus-isolates and one Actinomyces-isolates showed a four-fold MIC-increase as compared to the MIC of the respective parental strain. Culturing the five Streptococcus-isolates with eight-fold MIC-increase without selection pressure for up to 3 passages showed unaltered or even further increased (up to 16-fold) MICs.
Conclusions: Oral bacterial isolates show phenotypic adaptation upon repeated exposure to CHX and CPC, which was more pronounced in the CHX group. Further investigations are needed in order to reveal the genetic background behind these phenotypic adaptations.