Objective. The purposes of this study were to determine what microbial flora were present in
teeth after failed root canal therapy and to establish the outcome of conservative re-
treatment. Study design. Fifty-four root-filled teeth with persisting periapical lesions were
selected for re-treatment. After removal of the root filling, canals were sampled by means of
advanced microbiologic techniques. The teeth were then re-treated and followed for up to 5
years. Results. The microbial flora was mainly single species of predominantly gram-positive�…