Black-pigmented Bacteroides spp. in human apical periodontitis.

The incidence of black-pigmented (BP) Bacteroides spp. in 62 human dental root canal infections (35 acute and 27 clinically asymptomatic cases of apical periodontitis) in 57 adults was studied. Altogether 37 strains of BP Bacteroides were found in 31 infections, always in mixed anaerobic infections. Two different BP Bacteroides species were present in six infections. B. intermedius was most frequently isolated (15 of 62 canals; 24%) followed by B. denticola which was present in 12 cases. Asaccharolytic BP Bacteroides species, B. gingivalis and B. endodontalis, were found in eight cases. BP Bacteroides species were found both from symptomatic and asymptomatic infections, but there were also several symptomatic cases from which BP Bacteroides species were not isolated. B. gingivalis and B. endodontalis were present only in acute infections, B. intermedius was found both in symptomatic and asymptomatic infections, and B. denticola occurred mostly in asymptomatic infections. BP Bacteroides species were isolated initially from 9 of the 11 teeth with symptoms at 1 week, but only from 22 of the 51 teeth that were symptomless at 1 week. Two strains of B. denticola were resistant to penicillin G at a concentration of 2.4 micrograms/ml, but the MIC of penicillin G for all other strains was 0.6 micrograms/ml or lower. Forty-two randomly selected patients received penicillin V (oral administration, 650 mg, three times daily) during the first week of endodontic therapy. Penicillin had no effect on the occurrence of symptoms after 1 week compared with the control group (20 patients).