Changes in root canal microbiota during the development of rat periapical lesions

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Abstract
Periapical lesions are reproducibly induced in rats by pulp exposure and infection from the oral cavity. Lesions expand rapidly between day 7 and day 15‐20 (active phase), with slowed expansion thereafter. In the present study we characterized the root canal microbiota present during the active phase of lesion development in this system. The mandibular first molars of Sprague‐Dawley rats were exposed on day 0. The teeth were extracted after 7 days (n= 10 animals) and 15 days (n= 10), and the microbiota present in roots was isolated and characterized. The number of colonies isolated per tooth was similar on day 7 (1.53 ± 0.64 colony‐forming units × 103) and day 15 (1.49 ± 0.37 colony‐forming units × 103). No colonies were isolated from unexposed control teeth. Anaerobic bacteria increased significantly between day 7 (24.3 ± 5.7%) and day 15 (47.3 ± 7.5%), and the proportion of gram‐negative organisms increased from day 7 (24.3 ± 6.1) to day 15 (46.9 ± 6.8). The predominant bacteria included, on day 7: Streptococcus and Bacteroides species; on day 15: species of Streptococcus, Bacieroides, Prevotclla, Neisseria and Peptostreptococcus. Streptococcus oralis, Streptococcus mitis, Streptococcus rattus, Bacteroides pneumosinles and Bacteroides ureolyticus were frequently isolated at both points. Although approximately the same mean number of different species (∼3.5) was isolated per tooth on both day 7 and 15, the overall diversity of the isolates increased on day 15. These results demonstrate that the root canal microbiota becomes increasingly gram‐negative and anaerobic with the emergence of Peptostreptococcus, Bacteroides, Prevotella and Neisseria during the period of rapid lesion expansion in this model.