•Purpose: To evaluate the total bacterial counts and the presence, levels, and relative abundance of candidate endodontic pathogens exclusively in the apical root canal system associated with post-treatment apical periodontitis.
•Apical root specimens obtained during periradicular surgery of 27 adequately treated teeth with persistent apical periodontitis were cryogenically ground.
•DNA was extracted from the powder, and real-time polymerase chain reaction was used to quantify the total bacteria and 7 bacterial taxa.
•Most highlighted Results:
•Samples from 21 teeth were positive for bacteria.
•Streptococcus species were the most prevalent (76%) followed by members of the Actinobacteria phylum (52%) and Pseudoramibacter alactolyticus (19%).
•The mean total bacterial load in the apical root segments was 5.7 104 cell equivalents per root apex.
•Streptococci comprised from 0.02%–99.9% of the total bacterial counts, Actinobacteria from 0.02%–84.7%, and P. alactolyticus from 67.9%–99%.
•Although Enterococcus faecalis was found in only 3 (14%) cases, it was dominant in 2.
Streptococcus species, members of the Actinobacteria phylum, and P. alactolyticus were the most prevalent taxa in the apical canal system and dominated the bacterial populations in many cases of post-treatment apical periodontitis.