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-alpha-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 (mu g/ml) ranges were 32-64/64-128 for TR alone against all 10 MRSA isolates. Chequer-board 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 log(10) cfu/ml increase in killing), but the TR/LEV combination changed to additivity (1.839 log(10) 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. (C) 2011 Published by Elsevier GmbH.