4.2. Linezolid Susceptibility Testing

In order to determine the minimum inhibitory concentrations (MICs) against linezolid among the 34 isolates selected for further investigation, Agar dilution was used to reconfirm the MIC of the selected strains following CLSI Guideline M7, 11th edition [36]. The linezolid powder used to prepare agar plates was from Dong-A Pharmaceutical (Seoul, South Korea). The MIC range tested was from 0.25 μg/mL to 64 μg/mL. All MICs were interpreted according to CLSI M100-ED32. The isolates were determined as susceptible to linezolid when MIC ≤ 2 μg/mL, intermediate resistant to linezolid when MIC = 4 μg/mL, and resistant to linezolid when MIC ≥ 8 μg/mL. The isolates for which the linezolid MICs were greater than or equal to 4 μg/mL were defined as linezolid-nonsusceptible. E. faecalis ATCC 29212 and S. aureus ATCC 29213 served as quality control (QC) strains.