Alkaline Tolerance and Biofilm Formation of Root Canal Isolates of Enterococcus faecalis: An In Vitro Study

The aim of this study was to compare the effects of glucose and glycerol (provided as principal fermentable supplements) on alkaline tolerance and biofilm-forming capabilities of root canal–derived strains of Enterococcus faecalis and those from other environments. The planktonic growth kinetics and the biofilm-forming capabilities of E. faecalis isolates (identified by 16S ribosomal DNA sequencing) were compared when supplied with glucose and glycerol at pH levels of 8 and 11 in a microtiter plate. The metabolic activity of the biofilms plate that formed at a neutral pH level (supplied with either glucose or glycerol) was measured after subsequent adjustment to a pH level of 11. Ten isolates (7 from root canals and 3 from other sources) were examined. The lag phase and the doubling time increased under elevated alkalinity irrespective of either the fermentable supplement (glucose or glycerol) or the origin of the isolate. Biofilm formation and metabolic activity varied among strains, but neither was related to the source of isolation. In general, biofilm formation was enhanced when grown in glucose compared with glycerol and at a pH of 8 compared with a pH of 11 (irrespective of the fermentable supplement). The provision of glycerol did not increase either the planktonic growth rate or biofilm development compared with glucose but significantly increased the metabolic activity of biofilms, especially at a pH of 11 compared with a pH of 8. In the nutrient-deprived environment of a necrotic or root-filled root canal, glycerol may be an alternative energy source that can promote increased metabolic activity of E. faecalis under alkaline treatment conditions.