Objective: This report describes 2 endodontic patients who had persistent signs and symptoms after conventional root canal treatment. The aim of this study was to determine what microorganisms were present in the root canals of the teeth with failed endodontic therapy.
Study design: After removal of the root fillings, the canals were sampled by advanced microbiological techniques and the isolates were characterized by various tests.
Results: Bacteria, which grew in pure cultures, were isolated in each case. The bacteria were similar to each other and were classified as Actinomyces on the basis of phylogenic and phenotypic evidence. The bacteria were different from others within the genus, thus warranting designation as a new species, Actinomyces radicidentis.
Conclusions: The 2 cases of endodontic failure were infected with A radicidentis, a new Actinomyces species. This bacterium joins a restricted group of other microorganisms that have been associated with failure of root canal treatment.