Antibacterial and synergy of a flavanonol rhamnoside with antibiotics against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA)

The in vitro antibacterial activity of taxifolin-7-O-α-l-rhamnopyranoside (TR) and its synergy with four conventional antibiotics (ampicillin (AMP), levofloxacin (LEV), ceftazidime (CAZ) and azithromycin (AZM)) against ten clinical isolates of methicillin-resistant staphylococcus aureus (MRSA) were evaluated, respectively. Individual MICs and MBCs were determined by microdilution methods following the CLSI guidelines. Anti-MRSA synergy effects were measured using the chequerboard and time–kill curve tests. MICs/MBCs (μg/ml) ranges were 32–64/64–128 for TR alone against all 10 MRSA isolates. Chequerboard method showed that significant synergies were observed for the TR/CAZ and TR/LEV combinations with FICI ranged 0.187–0.375 and 0.25–0.5, respectively. Some synergy and additivity effects were also observed for TR/AMP and TR/AZM combinations. In the time–kill dynamic confirmation test, synergy results kept by the TR/CAZ combination (2.186 log10 cfu/ml increase in killing), but the TR/LEV combination changed to additivity (1.839 log10 cfu/ml increase in killing). These results demonstrated that TR enhanced the efficacy of CAZ and LEV in vitro, which had potential for combinatory therapy of patients infected with MRSA.