Amoxicillin-metronidazole disk for antimicrobial susceptibility testing of subgingival bacteria

Background: Antibiotics are used in periodontal therapy in selected cases, but therapy is rarely guided by antimicrobial susceptibility testing (AST). Direct AST of the oral microbiota using a combination disk with different antibiotics could provide a new way of AST to guide treatment planning. Methods: We performed AST of 46 strains of Aggregatibacter actinomycetemcomitans, Fusobacterium nucleatum complex, Prevotella species, and Porphyromonas gingivalis, with a combination disk of amoxicillin (AMX) and metronidazole (MET). The AMX-MET was compared to the largest inhibition zone diameter (IZD) obtained with AMX or MET disks, using an ordinary least square linear regression model. Results: The IZD of the AMX-MET correlated with the AMX for A. actinomycetemcomitans (interception 0.3) and with the MET for Fusobacterium (interceptions −1.25). For Prevotella, the AMX-MET was compared to AMX and MET after 20 and 44 h resulting in a superior correlation after 20 h (interception 0.06 vs 6.61 after 44 h). For P. gingivalis, the AMX-MET was compared to MET after 44 h resulting in an inferior correlation (interception 16.65). Conclusion: The IZD of AMX-MET was comparable to that of AMX and MET for important periodontal pathogens, which opens for studies on direct AST of oral samples with a mixed microbiota. Key message: The amoxicillin-metronidazole disk for antimicrobial susceptibility testing results in comparable inhibition zone diameters to that of AMX and MET for important periodontal pathogens.