Antagonistic, synergistic, and additive antibacterial interaction between ciprofloxacin and amoxicillin against Staphylococcus aureus

The aim of this in vitro study was to evaluate the interaction between ciprofloxacin and amoxicillin against beta‐lactamase‐producing Staphylococcus aureus. Concentration‐dependent curves for each individual drug were carried out to obtain the mean inhibitory concentration in the agar well diffusion assay. Then, different ratios of the ciprofloxacin–amoxicillin combination (0.5:0.5, 0.8:0.2, 0.2:0.8, 0.9:0.1, 0.1:0.9, 0.95:0.05, and 0.05:0.95) were assessed. Data were analyzed using the isobolographic analysis and interaction index. The isobolographic evaluation shows that the 0.9:0.1 and 0.95:0.05 ratios of the ciprofloxacin–amoxicillin combination produced a synergistic antimicrobial interaction, the 0.8:0.2, 0.2:0.8, 0.1:0.9, and 0.05:0.95 proportions showed an additive antibacterial effect, and the 0.5:0.5 proportion induced antagonistic antimicrobial effects. The interaction index showed similar outcomes to the isobolographic analysis. In conclusion, the data of this study mainly show antimicrobial additive results of the ciprofloxacin–amoxicillin combination against beta‐lactamase‐producing S. aureus.