4.3. Determination of Minimal Inhibitory Concentration

The present study tested the susceptibility of MSSA and MRSA strains by measuring the minimal inhibitory concentration (MIC). The MIC values of DMC and the β-lactam antibiotics OXA, AMP, and GEN against MRSA and MSSA were determined via broth microdilution assay. A series of 2-fold dilutions with an initial concentration of 1000 μg/mL of DMC were prepared in MHB and BHIB using a 96-well microplate. As for the three antibiotics, the initial concentration was 2000 μg/mL respectively. The inocula were adjusted to 1.5 × 10⁶ colony-forming unit (CFU)/mL. After incubating for 24 h at 37 °C, added MTT reagent and continued incubating for 30 min. The yellowish MTT solution was reduced to a dark blue formazan product by the mitochondrial dehydrogenases of living cells. The color depth is highly positively correlated with the number of live bacteria, which can visually display the minimum inhibitory concentrations.