Antimicrobial Susceptibility Testing Protocol

Species identification and antimicrobial susceptibility testing were performed using NMIC/ID4 card with BD Phoenix^TM100 Automated Microbiology System (Becton, Dickinson and Company, Sparks, Maryland, USA). The following antimicrobial agents were tested: amikacin (8 ~ 32 μg/mL), amoxicillin/clavulanate (4/2 ~ 16/8 μg/mL), ampicillin (4 ~ 16 μg/mL), ampicillin/sulbactam (4/2 ~ 16/8 μg/mL), aztreonam (2 ~ 16 μg/mL), cefepime (2 ~ 16 μg/mL), cefotaxime (1 ~ 32 μg/mL), ceftazidime (1 ~ 16 μg/mL), ciprofloxacin (0.5 ~ 2 μg/mL), gentamicin (2 ~ 8 μg/mL), imipenem (1 ~ 8 μg/mL), levofloxacin (1 ~ 8 μg/mL), meropenem (1 ~ 8 μg/mL), piperacillin/tazobactam (4/4 ~ 64/4 μg/mL), tetracycline (2 ~ 8 μg/mL), trimethoprim/sulfamethoxazole (0.5/9.5 ~ 2/38 μg/mL), cefotaxime/clavulanate (for ESBL, < 9 μg/mL), ceftazidime/ clavulanate (for ESBL, < 9 μg/mL), cefpodoxime-proxetil (for ESBL, < 9 μg/mL), ceftazidime (for ESBL, < 9 μg/mL) and ceftriaxone/clavulanate (for ESBL, < 9 μg/mL) [15 (link)]. If the MIC of ciprofloxacin was less than or equal to 0.5 μg/mL, or the MIC of levofloxacin was less than or equal to 1 μg/mL, sensitivity or intermediation was confirmed by disk diffusion test (ciprofloxacin (5 μg) and levofloxacin (5 μg), respectively). Phenotypic ESBL confirmation was performed with a double-disk synergy test (cefotaxime (30 μg), cefotaxime/clavulanic acid (30 μg/10 μg), ceftazidime (30 μg) and ceftazidime/clavulanic acid (30 μg/10 μg) disk) following clinical and laboratory standards institute (CLSI) criteria [16 ]. The recently revised CLSI species-specific clinical breakpoints (CBPs) were used to interpret the MIC results. The quality control strains were Escherichia coli ATCC25922, Pseudomonas aeruginosa ATCC27853 and Klebsiella pneumoniae ATCC700603.