LAUSR

Introduction: The root canal microbiota in root‐filled teeth with post‐treatment apical periodontitis before and after chemomechanical instrumentation and irrigation with either 1% sodium hypochlorite (NaOCl) or 2% chlorhexidine digluconate were analyzed by using the pyrosequencing method. Methods: Samples from 10 root‐filled teeth with apical periodontitis undergoing retreatment were taken before (S1) and after (S2) preparation using irrigation with either NaOCl (n = 5) or 2% chlorhexidine digluconate (n = 5). DNA was extracted, and the 16S rRNA gene (V3‐V5) variable regions were amplified and subjected to pyrosequencing (GS junior 454) to determine the bacterial composition. Results: Pyrosequencing yielded 43,797 sequence reads in S1 and 9196 in S2 samples. Overall, 125 bacterial species belonging to 68 genera (S1, 59; S2, 38) and 9 phyla were found. The most abundant and prevalent phyla in S1 and S2 samples were Firmicutes, Fusobacteria, Bacteroidetes, and Actinobacteria. The most represented, abundant, and prevalent genera in S1 and S2 samples were Streptococcus and Fusobacterium. The most prevalent species in S1 and S2 samples were Fusobacterium nucleatum ss. vincentii, Streptococcus oralis/mitis, Streptococcus intermedius, and Streptococcus gordonii. The mean number of species per root canal was 20 (range, 4–37) in S1 and 9 (range, 4–15) in S2, respectively. Conclusions: A high interindividual diversity was observed in both S1 and S2 samples, with no difference between the two irrigation groups. F. nucleatum ss. vincentii and some Streptococcus species were the most prevalent species in pre‐preparation and post‐preparation samples during retreatment of root‐filled teeth with infection.