Determination of MICs

A two-fold broth microdilution method was used to determine the MICs of antimicrobials, following the guidance of the CLSI (M100) (Clinical and Laboratory Standards Institute, 2018). The MIC was defined as the lowest concentration of an antimicrobial agent that inhibits more than 90% bacterial growth from three independent tests, or as the lowest concentration with no visible bacterial growth ( Table S2 ). A positive growth control, which contained bacterial suspensions in cation-adjusted Mueller-Hinton broth (CAMHB) without antimicrobials, and a negative control with broth only were included in a 96-well plate with a final volume of 200 μL. The plate was incubated for 20-24 h at 35°C in ambient air. E. coli ATCC 25922 was used as a quality control strain. The inhibition rate was calculated using the equation (1):

Up to now, there are no CLSI susceptibility breakpoints for tigecycline and sulbactam against A. baumannii. Therefore, we used the CLSI criterion of ampicillin/sulbactam combination for sulbactam breakpoint and the FDA criterion for tigecycline breakpoint (i.e. MIC value ≤ 2 mg/L, susceptible; 4-8 mg/L, intermediate; ≥ 8 mg/L, resistant) (Navon-Venezia et al., 2007).