The purpose of this study was to evaluate the susceptibilities fo endodontic infectious flora to various antibiotics. Totally, 103 cultivable isolates from 111 patients' teeth with persistent periapical radiolucent lesions were assessed. After removal of the root-filling material, the canals were sampled with paper points. Samples were grown under anaerobic conditions. The E-test was used to determine the susceptibility. Results showed that the microbial flora was predominantly facultative gram-positive organisms and anaerobic gram-negative bacilli. The antibiotic susceptibility tests showed that ampicillin resistance was found with very high minimum inhibitory concentrations (MICs) for the isolated strains of Fusobacterium nucleatum, Enterococcus, Peptostreptococcus micros, and Prevotella intermedia. Metronidazole and clindamycin retained their killing activity against anaerobes, but emerging resistance has beome a problem in some clinical settings. Tetracycline, erythromycin, and cephalosporin were considerably less active against the majority of the tested bacterial strains. Meanwhile minocycline and doxycycline exerted strong antimicrobial activities, and they could inhibit strains at very low concentrations; trovafloxacin appears to be a promising drug expressing the highest activities (MIC□ ≤ 1.0μg/ml), against a wide variety of endodontic infectious flora. In conclusion, the in vitro activities of commonly used antibiotics against clinical endodontic microflora have decreased remarkably in Taiwan, and this should raise concern.
