A preliminary study investigating the survival of tetracycline resistant Enterococcus faecalis after root canal irrigation with high concentrations of tetracycline

Aim To compare the ability of two Enterococcus faecalis strains to survive exposure to an irrigation solution containing a high concentration of tetracycline in the root canals of bovine teeth. Methodology The root canals of twelve bovine incisor root sections were chemo-mechanically prepared using commercially available drills, sodium hypochlorite and ethylenediamine tetra-acetic acid. The root sections were divided into two groups and inoculated with either a tetracycline sensitive or resistant strain of E. faecalis. The strains are isogenic, however one contains a conjugative transposon related to Tn916 which confers resistance to tetracycline, and the other strain is sensitive to the antibiotic. After 26 days of incubation the root canals were irrigated using one of three solutions (sterile distilled water, 50% ethanol or tetracycline at a concentration of 30 mg mL-1). The roots were sampled by grinding dentine and canal contents and the debris collected were incubated in broth to assess growth. Results Irrigation with sterile distilled water or 50% ethanol did not remove all of the cells present. The tetracycline containing solution was efficient in preventing any growth of sensitive E. faecalis, however the resistant strain was able to survive a 5 min exposure at 30 mg mL-1. Conclusions The presence of the Tn916-like conjugative transposon containing the tetracycline resistance gene tet(M) allowed an E. faecalis strain to survive irrigation using a solution containing an extremely high concentration of tetracycline in a root canal model.