Bacterial isolates and antimicrobial susceptibility testing

In this study, clinical A. baumannii isolates from BSI (n = 17) and UTI (n = 16) were collected from Huashan Hospital in Shanghai between 2016 and 2017. Isolates from HAP (n = 31) were from a domestic thirteen-centre clinical study on colistin methanesulfonate (registration number: {"type":"clinical-trial","attrs":{"text":"NCT01940731","term_id":"NCT01940731"}}NCT01940731) including Hunan People’s Hospital, Shanghai 10th People’s Hospital, The Second Hospital of Jilin University, West China Hospital of Sichuan University, etc. across China. Colistin sulfate (lot number SLBD8306V; Sigma-Aldrich, St Louis, MO), polymyxin B (lot number R046V0; USP), minocycline, doxycycline, sulbactam, meropenem, aztreonam (lot numbers: 130514–200401, 130485–201703, 130430–201408, 130506–201403 and 130507–201303, respectively; National Institutes for Food and Drug Control, Beijing, China), tigecycline (lot number: 10-MWC-62-1; USP) and doripenem (lot number 0379; Shionogi & Co Ltd) were used in this study. The minimum inhibitory concentrations (MICs) of 64 isolates were determined using the microbroth dilution method for three replicates in one batch with Escherichia coli ATCC25922 as the quality control. A total of nine antimicrobials were included and the results were interpreted referring to the CLSI where possible (CLSI M100 2020). CLSI does not currently provide breakpoints for tigecycline, sulbactam and aztreonam. FDA-recommended criteria for Enterobacteriaceae were used for tigecycline susceptibility (≤ 2, 4, ≥ 8 mg/L) [35].