Metronidazole plus Amoxicillin Activity on Antibiotic-Resistant Aggregatibacter actinomycetemcomitans

Objectives: Metronidazole plus amoxicillin exerts in vitro synergism against Aggregatibacter (Actinobacillus) actinomycetemcomitans (Aa). This interaction has been studied on a limited number of A. actinomycetemcomitans strains exhibiting a range of in vitro susceptibility to each of the two antibiotics. This study examined the effects of metronidazole plus amoxicillin on periodontal Aa isolates resistant in vitro to both metronidazole and amoxicillin. Methods: Aa was isolated on selective TSBV medium from subgingival plaque samples of 26 adults and adolescents with severe periodontitis. These isolates were resistant in vitro to 16 µg/ml of metronidazole and 8 µg/ml of amoxicillin (representing United States Clinical Laboratory Standards Institute non-susceptible breakpoint concentrations). Isolates were plated onto enriched Brucella blood agar supplemented with 16 µg/ml of metronidazole plus 8 µg/ml of amoxicillin, followed by anaerobic incubation. Aa growth on the metronidazole plus amoxicillin antibiotic-supplemented media was scored on a 0-3 scale. Results: 14 of 26 (54%) Aa strains resistant to both 16 µg/ml of metronidazole alone and 8 µg/ml of amoxicillin alone were totally inhibited in vitro by simultaneous exposure to the two antibiotic drug concentrations. An additional 7 (27%) antibiotic-resistant Aa strains demonstrated only trace growth levels (growth score = 1), and 5 (19%) strains were unaffected (growth score = 3), by the antibiotic combination. Conclusions: Therapeutic concentrations of metronidazole plus amoxicillin together inhibit or suppress a large proportion of clinical periodontal Aa isolates that otherwise exhibit in vitro resistance when exposed to the two antibiotic drugs individually. These findings further support the potential clinical value of metronidazole plus amoxicillin combination drug therapy for Aa-associated periodontal infections which resist subgingival eradication by conventional mechanical forms of periodontal therapy.