Fig. 1

Synergistic antimicrobial effects of oxacillin and NPPB against MRSA strains. (a) The effects of oxacillin and NPPB on the growth of five MRSA strains. The combined effect of oxacillin and NPPB on the growth MRSA strains was evaluated using the checkerboard assay. MICs of oxacillin, NPPB, and oxacillin plus NPPB were determined. The fractional inhibitory concentration index (FICI) ≤ 0.5 indicated synergism. The MICs of oxacillin against five MRSA strains were significantly reduced when combined with NPPB (10 µg/mL) compared to oxacillin alone. Green color indicates the MIC of oxacillin when administered alone, whereas pink represents the MIC of oxacillin in the presence of NPPB. (b) Time-dependent growth inhibition of the combination of oxacillin and NPPB on MRSA ATCC 33591. The bacteria were cultured in a 37℃ incubator at 200 rpm shaking overnight. The bacterial solution 1000-fold diluted in fresh TSB media was inoculated in 96-well plates with oxacillin (32 µg/mL), NPPB (10 µg/mL), or a combination of oxacillin (32 µg/mL) and NPPB (10 µg/mL). Absorbance at 630 nm was measured at different time points (2, 4, 6, 8, 10, and 24 h). The combination treatment significantly reduced bacterial growth over time compared with the individual treatment.