Antimicrobial susceptibility testing of Aggregatibacter Actinomycetemcomitans can be performed using the EUCAST medium for anaerobic bacteria

Background: The oral bacterium Aggregatibacter actinomycetemcomitans (Aa) is associated with infectious diseases treated with antibiotics, but specific recommendations for antimicrobial susceptibility testing (AST) of the species do not yet exist. Objective To evaluate the possibility to perform AST of Aa following EUCAST guidelines using FAA-HB and 20 h incubation. Method: Twenty-nine Aa strains were analysed using disk diffusion on FAA-HB media from three manufacturers. Plates with with ampicillin and azithromycin disks were incubated in 5% CO₂ and anaerobic conditions and examined after 20 and 44 h. Strains were additionally tested with the agar dilution method, and inhibition zone diameters (IZD) were correlated to the minimal inhibitory concentrations. Results: FAA-HB agar supported sufficient growth of Aa after 20 h incubation in 5% CO₂. No significant differences in IZD were found between the three media tested. Prolonged incubation (44 h) increased IZD, but 20 h incubation was adequate for reproducible results. Incubation in 5% CO₂ was superior to anaerobic conditions. Conclusion: Incubation in 5% CO2 and reading after 20 h incubation resulted in the best correlation between FAA-HB media from three manufacturers. The EUCAST disk diffusion method with FAA-HB for AST of Aa may be crucial for future standardization and development of clinical breakpoints for this species.