Antibacterial effects of peonol in combination with amoxicillin against Streptococcus pneumonia clinical isolates

Drug resistance is a crisis among major pathogens such as Streptococcus pneumonia (S. pneuomonia). The aim of this study was assessment of synergistic effects of amoxicillin and paeonol checkerboard assay on S. pneumonia. S. pneumonia isolates were resistant to several antibiotics. By combining peonol with amoxicillin, the MIC for amoxicillin alone decreased significantly compared to that of stand-alone amoxicillin. For example, the MIC for amoxicillin when combined with peonol ranged from 0.015 to 0.5 μg/mL, which corresponds to an approximate 8-16 fold decrease of amoxicillin MIC in some samples. Likewise, the average MIC of peonol decreased from approximately 2 mg/mL to an MIC of 0.125 mg/mL when combined with amoxicillin. The results calculated by FIC indices for all bacterial isolates that were exposed to both compounds demonstrated that 15 (75%) of bacteria tested indicated synergy (0.5≤FIC); thus indicating the development of strong synergistic effects between peonol and amoxicillin, while 5 (25%) indicated addition (1≥FIC>0.5) and no strain showed indifference (>1 FIC). No strains showed antagonistic interactions. Therefore, paeonol had good synergistic effect with amoxicillin.