Susceptibility of multidrug-resistant Aggregatibacter actinomycetemcomitans in vitro to amoxicillin-metronidazole and ciprofloxacin.

Aggregatibacter actinomycetemcomitans (Aa) is an important pathogen in human periodontitis, peri-implantitis and some non-oral infections. Simultaneous application of amoxicillin-metronidazole is synergistic against many Aa strains, including one resistant to both antibiotics individually. The present study evaluated the in vitro activity of amoxicillin-metronidazole in combination, as well as ciprofloxacin, on additional Aa clinical isolates resistant to both amoxicillin and metronidazole individually. A total of 26 Aa strains resistant to amoxicillin and metronidazole individually were recovered by selective culture from subgingival biofilm samples in United States patients with severe periodontitis. The clinical isolates were plated onto enriched Brucella blood agar supplemented with 8 mg/L of amoxicillin plus 16 mg/L of metronidazole to test for enhanced antimicrobial activity by the two antibiotics together, and onto Haemophilus test medium for ciprofloxacin (5 μg) disk diffusion testing. Among the 26 Aa study strains, 14 (53.9 %) were fully inhibited in vitro by the combination of amoxicillin-metronidazole, despite their resistance to both antibiotics individually. Partial inhibition by amoxicillin-metronidazole was found with 7 (26.9 %) Aa strains, whereas 5 (19.2 %) strains were unaffected by the drug combination. All 26 Aa study isolates were susceptible to ciprofloxacin. In conclusion, amoxicillin-metronidazole in combination exerted enhanced antimicrobial activity with total growth inhibition in vitro of approximately one-half of 26 Aa clinical isolates resistant to amoxicillin and metronidazole individually. All of the Aa study strains were susceptible in vitro to ciprofloxacin. These findings further support amoxicillin-metronidazole and ciprofloxacin drug therapies against susceptible Aa in oral and non-oral infections.