The evolving new understanding of endodontic infections

The science of oral microbiology is in a period of change from the era of bacterial cultivation to an era of molecular genetic methods and techniques. Already a significant body of new knowledge exists with regard to the oral flora in health and disease. Inevitably, this new knowledge has led to a better understanding of many oral diseases. In endodontics, the prevailing concepts are still to a great extent based on the results of the classical cultivation studies. However, a few groups have started to use molecular methods, and a new understanding of endodontic infections is presently evolving. Thus, the root canal infection clearly is more complex than revealed by cultivation methods alone, and both previously unidentified and uncultivable microorganisms have been detected by molecular methods. A reasonable estimate at present is that the infected root canal contains, not less than 10, but rather between 10 and 50 bacterial species which coincide well with the number of bacterial species normally found in a dental plaque sample and at different sites in the oral cavity. A further interesting finding in the studies using molecular techniques is that the microbiota of the infected root canal appears to be very similar to the flora of the periodontal pocket in patients with active periodontal disease. With regard to infection of periapical lesions in patients with asymptomatic apical periodontitis, electron microscopic and molecular methods have confirmed our cultivation findings that this is a common occurrence. Mature biofilms have been demonstrated on the external surfaces of root tips and in the form of sulfur granules within periapical granulomas. As in dental plaque, Actinomyces species appear to have a special role as scaffold builders in the development of sulfur granules. Other bacteria are then attracted to the site and a multibacterial granule (biofilm) develops. In addition, in situ hybridization studies show a variety of different bacteria and bacterial morphotypes in periapical lesions. With DNA–DNA hybridization between 11and 39 bacterial species have been recognized in the lesions, again confirming that in patients with active disease, the microbiotas of endodontic and periodontal infections are very similar. Thus, the recent findings demonstrate and confirm that the periapical endodontic lesion is not as hostile to microorganisms as many have thought. As clinicians we have to understand and accept that an infection might not be limited to the root of the tooth, but include the periapical lesion as well.