Additive or synergistic antimicrobial effects of amoxicillin and metronidazole on whole plaque samples: a preliminary report.

OBJECTIVE: In vitro data on the susceptibility of oral bacteria to the combination of metronidazole and amoxicillin is limited. 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: Prior to any treatment procedures subgingival plaque samples from patients with severe generalized periodontitis were taken. Appropriate dilutions were plated on Columbia blood agar supplemented with the following 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 at 36°C for 14 days and the colony forming units (CFU) were determined. RESULTS: Both applied metronidazole concentrations were able to decrease the CFU counts by approximately one order of magnitude in a log10 scale. Amoxicillin 3 μg/mL revealed a reduction of 2.4 log 10 CFU, 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 the combination of amoxicillin and metronidazole even at the lower antibiotic concentrations. CONCLUSION: Susceptibility screening of subgingival samples to metronidazole and amoxicillin and to their combination seems to offer a rational basis for the selection of adjunctive antibiotic therapy.