In Vitro Evaluation of the Antibacterial Activities of MTAD in Combination with Nisin against Enterococcus faecalis

Introduction: MTAD is a common intracanal irrigant. Although MTAD inhibits Enterococcus faecalis in root canals, its bactericidal effect against E. faecalis remains to be improved. Nisin, an antibacterial peptide, possesses a strong bactericidal effect. The study evaluated the synergetic action between MTAD and nisin against E. faecalis. Methods: The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were used to measure the antibacterial activities of MTAD, MTAN (substitution of doxycycline with nisin), and MTADN (nisin in combination with doxycycline), respectively. The synergetic effect between nisin and doxycycline was evaluated by fractional inhibitory concentration (FIC) and time-killing curves. Furthermore, morphologic changes in E. faecalis were observed by scanning electron microscopy after E. faecalis was treated with MTAD, MTAN, or MTADN for 24 hours. Results: The MBC of MTADN against E. faecalis was lower than that of MTAD and MTAN. The combination of nisin and doxycycline had a significantly synergetic antibacterial effect on E. faecalis. Among the 3 antimicrobial treatments, MTADN caused the most severe damage to E. faecalis. Conclusions: The combination of nisin and doxycycline has a synergetic antibacterial effect on E. faecalis, and MTAD in conjunction with nisin inhibits E. faecalis better than MTAD alone. (J Endod 2011;37:1116-1120)