Aim: Antibiotics can control bacterial infections, but their overuse can lead to resistance development in bacteria. Treatment of multi-drug resistant bacteria is always challenging. This study investigated whether a combination of antibiotics could increase the susceptibility of ocular isolates of Staphylococcus aureus and Streptococcus viridans strains.
Methods: The minimum inhibitory concentration (MIC) of quinolone ciprofloxacin and macrolide azithromycin were determined by the micro broth dilution method against the multi-drug resistant S. aureus 117 and ciprofloxacin-resistant S. viridans. Based on their MICs, combinations of these two isolates were analyzed in a checkerboard assay which provided fractional inhibitory concentration index (FICI). FICIs of ≤ 0.5 indicates synergism; 0.6-1 indicate additive effect; >1-4 - no interaction and ≥4 - antagonism.
Results: Both ciprofloxacin and azithromycin had a MIC of 8µg/ml for the multi-drug resistant S. aureus strain 117. The FICI for the combination was 0.2 for S. aureus 117 indicating synergism and 1 for S. “viridans” indicating additive effect. When antibiotics were combined, only 1.25µg/ml of ciprofloxacin and 0.39 µg/ml of azithromycin were needed to kill S. aureus 117, whereas 2.5µg/ml ciprofloxacin and 0.781µg/ml needed to kill S. viridans SV04.
Conclusion: The combination of ciprofloxacin and azithromycin enhanced their antimicrobial activity producing synergism for the multidrug-resistant strain of S. aureus. This may be useful for clinicians who are treating bacterial infections. Future research will be extended to include more pathogenic strains and investigate the mechanism of interactions.