Clinical studies on aggressive periodontitis have revealed improved outcomes for mechanical biofilm removal combined with metronidazole and amoxicillin. In-vitro information on the susceptibility of oral bacteria of this combination is hitherto scarce. The aim of this preliminary study was to determine the susceptibility of whole subgingival plaque-samples to amoxicillin and metronidazole and to their combination.
Methods:
Subgingival plaque samples of untreated patients with severe –by age- advanced periodontitis were selected for analysis. For the determination of the total anaerobic bacterial count 100µl of the dilutions were plated on Columbia blood agar plates supplemented with the following concentrations of the respective antimicrobial agents: 3 µg /ml amoxicillin, 8 µg /ml amoxicillin, 8 µg /ml metronidazole, 16 µg /ml metronidazole, 3 µg /ml amoxicillin plus 8 µg /ml metronidazole or 8 µg /ml amoxicillin plus 16 µg /ml metronidazole. All plates were incubated anaerobically for 14 days and the Colony forming units were determined.
Results:
The total anaerobic counts of the samples varied within 3.1 x 10-6 and 7.2 x 10-7. The percentage of black-pigmented colonies varied within a range from 40-80% of the respective total count. All samples showed resistance against metronidazole indicated by a decrease of bacterial growth on agar by approximately 1 log with both concentrations of the antibiotic agent. Similarly, all agar plates supplemented with 3 µg/ml amoxicillin showed bacterial growth with a mean log reduction of 2.4, whereas 50% of the samples did not grow on the plates supplemented with 8 µg/ml of amoxicillin. There was no anaerobic bacterial growth on agar plates supplemented with a combination of amoxicillin and metronidazole even at the lower antibiotic concentrations.
Conclusion:
Susceptibility screening using whole subgingival samples to metronidazole and amoxicillin and to their combination seems to offer a rational diagnostic tool for the selection of adjunctive antibiotic therapy..