Florfenicol

The minimal inhibitory concentrations (MICs) of 17 antimicrobials—namely, meropenem, cefotaxime, ceftiofur, ampicillin, ciprofloxacin, enrofloxacin, levofloxacin, nalidixic acid, amikacin, gentamicin, kanamycin, streptomycin, tigecycline, doxycycline, tetracycline, florfenicol and fosfomycin—for these isolates were determined by the agar dilution method following the guidelines of the Clinical and Laboratory Standards Institute [28 ]. The MIC of colistin was determined according to the method of 2017 EUCAST (available at http://www.eucast.org/clinical_breakpoints/ (accessed on 29 June 2018)). The resistant breakpoints for colistin and tigecycline were recommended by the 2017 EUCAST (http://www.eucast.org/clinical_breakpoints/ (accessed on 28 July 2018)), while the breakpoints for the remaining antimicrobials were recommended by the CLSI [28 ,29 ]. E. coli ATCC 25,922 was used as the control strain. Ceftiofur, florfenicol and tigecycline were from Solarbio Life Sciences Co. (Beijing, China) and the remain antimicrobials used in this study were purchased from China National Institutes for Drug Control (Beijing, China).

Antimicrobial susceptibility testing (AST) was performed by broth microdilution method as our previously described (Liu et al., 2021b (link)). Briefly, 0.5 McFarland inoculum suspensions were diluted 1:100 to a final inoculum density of 10⁶ CFU/ml. Ampicillin (AMP), ceftazidime (CAZ), cefotaxime (CTX), aztreonam (AZM), tetracycline (TET), doxycycline (DOX), ciprofloxacin (CIP), enrofloxacin (ENR), gentamicin (GEN), sulfamethoxazole (SMZ), fosfomycin (FOS), florfenicol (FFC), colistin (COL), and meropenem (MEM) (Solarbio, Beijing, China) were selected to determine the minimum inhibitory concentrations (MICs) of MNPECs. The MICs were recorded as the lowest concentrations of antimicrobials in the wells, where no visible bacteria growth was observed. Escherichia coli ATCC 25922 was used as standard reference strains.