Microbiota of periapical lesions refractory to endodontic therapy.

The periapical microbiota of 36 teeth with refractory apical periodontitis was investigated. None of the teeth had responded to conventional endodontic or long-term (> 6 months), calcium-hydroxide treatment. Eight patients had received antibiotics systemically. After anaerobic culture, a total of 148 microbial strains were detected among 67 microbial species. One of the 36 lesions was culture-negative. Approximately half (51.0%) of the bacterial strains were anaerobic. Gram-positive species constituted 79.5% of the flora. Facultative organisms, such as Staphylococcus, Enterococcus, Enterobacter, Pseudomonas, Stenotrophomonas, Sphingomonas, Bacillus, or Candida species were recovered from 27 of the lesions (75%). Sulfur granules were found in 9 lesions (25%). In these granules Actinomyces israelii, A. viscosus, A. naeslundii, and A. meyeri were identified. Other bacterial species, both gram-positive and gram-negative, were detected in the granules as well. Two sulfur granules did not contain Actinomyces. Scanning electron microscopy demonstrated rod- and spirochete-like cells in the granules, and transmission electron microscopy revealed organisms with copious amounts of extracellular material. Outer membrane vesicles were also seen. Some of the granules were calcified. This study demonstrated a wide variety of microorganisms, particularly gram-positive ones, in the periapical lesions of teeth with refractory apical periodontitis.