Determination of MIC of amoxicillin among clinical strains of Aggregatibacter actinomycetemcomitans from various geographic regions endorse the continuous use of this drug for treatment of periodontitis

Aim: The combination of amoxicillin and metronidazole is used in the treatment of Aggregatibacter actinomycetemcomitans (Aa)-associated periodontitis in some patients. Aa is innately susceptible to betalactam agents, but resistance or decreased susceptibility has been reported. Elucidation of the prevalence and mechanisms of resistance to betalactams in Aa is important in order to treat periodontitis patients with the most optimal antibiotic regimen. Method: 259 Aa strains were collected from patients with geographically diverse origin and serotyped with a multiplex PCR. Minimal inhibitory concentration (MIC) values were determined using the agar dilution method, with amoxicillin (range 0.00 mg/L to 8.00 mg/L) incorporated into 5 % blood agar plates with 20 mg/mL NAD. The plates were inoculated with 1 μL bacterial suspensions (10(4) CFU/ml), incubated in 5 % CO(2) at 37 C°, and visually inspected after 24 and 48 hours. A MIC ≥ 4.00 mg/L was categorised as resistant, using EUCAST interpretative criteria for Haemophilus species. Results: None of 259 clinical Aa strains exhibited resistance towards amoxicillin. MIC-values varied from 0.25 mg/L to 2.00 mg/L. There was no clear correlation between MIC-value and serotype. Conclusion: The study showed no contraindications to the continuous use of amoxicillin as a part of the treatment strategy in patients with Aa-associated periodontitis.