Antimicrobial efficacy of chlorhexidine against bacteria in biofilms at different stages of development

Detailed knowledge on the nature of the physiological and metabolic phases of biofilm development is important in combating resistant, disease-associated biofilms. The aim of this study was to examine the susceptibility of multispecies biofilms at different phases of growth to root canal irrigants.

The multispecies biofilms were grown from plaque bacteria on collagen-coated hydroxyapatite discs in brain-heart infusion broth for time periods ranging from 2 days to several months. Fresh nutrients were added weekly for the first 3 weeks, followed by a nutrient deprivation phase, by adding fresh brain-heart infusion broth medium only once a month. Biofilms of different age were subjected to 1-, 3-, or 10-minute exposure to 2% chlorhexidine (CHX) or CHX-Plus. After treatment, the volume ratio of dead bacteria in biofilms was assessed by confocal laser scanning microscopy by using a LIVE/DEAD viability stain. Biofilm structure was visualized by scanning electron microscopy.

The thickness of biofilms increased from 57 μm (2 days) to 155 μm (3 weeks) during biofilm development. It reached a steady state under nutrient-limiting conditions, with thickness of 190 μm (6 weeks) to 201 μm (12 weeks). The proportion of killed bacteria in mature biofilms (3 weeks) was lower than in young biofilms (2 days, 1 and 2 weeks) after treatment with both CHX products (P < .01).The resistance of mature biofilms under the nutrient-limiting phase (6-12 weeks) to CHX remained stable and was similar to 3-week-old biofilm. However, treatment with CHX-Plus for 3 and 10 minutes did not lose effectiveness against biofilm bacteria in mature and nutrient-limited phases. CHX-Plus showed higher levels of bactericidal activity at all exposure times than 2% CHX (P < .01).

Bacteria in mature biofilms and nutrient-limited biofilms are more resistant to CHX killing than in young biofilms. The results emphasize the importance of standardization of factors such as biofilm age when studying the comparative effectiveness of disinfecting agents against biofilm bacteria.