Abstract: A recognized pathogen in post- treatment endodontic infections, Enterococcus faecali（s E. faecalis）is frequently isolated both in mixed flora and in monocultures. E. faecalis is rarely present in primary apical periodontitis,
but it is the dominant microorganism in root - filled teeth presenting with post - treatment apical periodontitis. It is probably the species that can best adapt to and tolerate the ecologically demanding conditions in the filled root canal. Eradication of E. faecalis from the root canal with chemomechanical preparation and using disinfecting irrigants and antibacterial dressings remains a challenge.