Antibiotic resistance of subgingival species during and after antibiotic therapy
Authors: Feres M. , Haffajee A. D. , Allard K. , Som S. , Goodson J. M. , Socransky S. S. 
Published: December 2004

Aim: The purpose of the present investigation was to determine the percentage and identity of antibiotic‐resistant species in subgingival plaque and saliva samples from chronic periodontitis patients treated by scaling and root planing followed by orally administered amoxicillin or metronidazole.

Method: In all, 20 chronic periodontitis patients were selected for study. After clinical and microbiological monitoring, subjects were randomly assigned to receive either orally administered amoxicillin at the dosage of 500 mg, 3 times daily for 14 days or orally administered metronidazole at the dosage of 250 mg, 3 times daily for 14 days. For the antibiotic resistance determinations, subgingival plaque samples were taken from six posterior teeth at baseline, and 90 days; and from two randomly selected teeth at 3, 7 and 14 days during and after antibiotic administration. Samples were plated on enriched blood agar plates with or without either 2 µg/mL metronidazole or 2 µg/mL amoxicillin. Colonies were counted at 7 days. Significant differences in percentage of resistant organisms over time were determined by the Quade test. Microbial growth was washed from antibiotic‐containing media and the identity of species determined using checkerboard DNA–DNA hybridization. Data were compared with those obtained in a previous study from subjects receiving SRP only or SRP followed by 14 days of orally administered doxycycline. The level of doxycycline used to determine antibiotic resistance in that study was 4 µg/mL.

Results: The mean percentage of resistant isolates increased during antibiotic administration and returned to baseline levels by 90 days post therapy. The mean percentages (± SEM) of isolates resistant to 2 µg/mL metronidazole were 53 ± 9, 65 ± 9, 79 ± 4 and 69 ± 7 at baseline, 3, 7 and 14 days during antibiotic administration, and 57 ± 4, 64 ± 5, 62 ± 7 and 47 ± 6 at 3, 7, 14 and 90 days after antibiotic administration. At the same time points, the percentage of resistant isolates to amoxicillin was 0.5 ± 0.2, 22 ± 12, 14 ± 5 and 37 ± 11 during, and 31 ± 11, 8 ± 3, 3 ± 2 and 3 ± 0.6 after, administration. Antibiotic‐resistant isolates of resistant species detected during or after therapy were also detected prior to therapy. The most prevalent resistant species in the metronidazole‐treated group were: A. naeslundii 1, S. constellatus, A. naeslundii 2, S. mitis, S. oralis, A. odontolyticus, S. sanguis, and in the amoxicillin‐treated group: S. constellatus, P. nigrescens, E. saburreum, A. naeslundii 1, S. oralis, P. melaninogenica and P. intermedia.

Conclusions: Systemic antibiotic administration transiently increased the percentage of resistant subgingival species, but a major component of subgingival plaque remained sensitive to the agents during their administration. Antibiotic‐resistant isolates of resistant species could be detected in samples both prior to and after therapy. However, % antibiotic‐resistant isolates returned to baseline levels 90 days after antibiotic administration.