The effect of MTADN on 10 Enterococcus faecalis isolates and biofilm: an in vitro study

Enterococcus faecalis is frequently isolated from both secondary and persistent root canal infections. MTAD is an effective intracanal irrigant, but its bactericidal activity requires improvement. The goal of this study was to investigate whether nisin, an antibacterial peptide, improves the antibacterial and antibiofilm activities of MTAD against E. faecalis clinical isolates.

Ten E. faecalis strains were isolated from root canals with persistent periapical lesions. The antibacterial activity of MTAD, MTADN (nisin in combination with doxycycline), and MTAN (nisin in place of doxycycline) against E. faecalis isolates was evaluated using minimal inhibitory concentration and minimal bactericidal concentration assays. Scanning electron microscopy was used to observe E. faecalis biofilms on the chamber surface of the root canal, and confocal laser scanning microscopy was used to show the antibiofilm activity of the 3 irrigants.

Ten E. faecalis strains were successfully isolated from clinical samples by selective medium, colony morphology, oxygen tolerance, bile resistance, Gram staining characteristics, and polymerase chain reaction identification. The antimicrobials MTAD, MTADN, and MTAN had different antibacterial activities against the 10 E. faecalis isolates, with MTADN showing superior antibacterial efficacy compared with MTAD. Strain D had the greatest sensitivity to MTAD and MTADN, whereas strain E was the most resistant to these compounds. Strain E was able to form a mature biofilm with complex architecture. Importantly, confocal laser scanning microscopic images showed that MTADN was able to kill completely E. faecalis isolates grown as a biofilm, whereas MTAD was unable to do so.

MTADN effectively inhibited both the growth of E. faecalis root canal isolates and these isolates' biofilms. These results are encouraging and suggest that MTADN has a considerable potential for use as an effective irrigant.