Synergistic Evaluation of Compounds against MRSA

The synergy of the two compounds and oxacillin (Sigma–Aldrich, St. Louis, Missouri, USA) against MRSA S. aureus CH 10850 was evaluated by the checkerboard assays [35 (link)], performed using 2-fold increasing concentrations of both compound (from 8 to 0.125µg/mL for compound 1 and from 512 µg/mL to 8 µg/mL for compound 2) and oxacillin (from 512 to 0.5 µg/mL). Since the two compounds were resuspended in DMSO, the upper limit of the concentrations range tested was determined considering a final concentration of 1% DMSO. The combinations of each compound and oxacillin were evaluated by fractional inhibitory concentration (FIC) index, interpreted as follows: ≤0.5, synergy; > 0.5 and ≤ 1.0, additive; >1.0 and <4, indifferent and ≥4, antagonistic.