Ceftiofur

Shikimic acid and all antibiotics, including oxacillin, amoxicillin, ampicillin, ceftiofur, carbenicillin, and cefoxitin, were purchased from Macklin (Shanghai, China). PI, DAPI, the ATP Assay Kit, the ROS Assay Kit, and the Alkaline Phosphatase Assay Kit, were purchased from Beyotime (Shanghai, China). The ELISA Kit was purchased from Elkbiotechnology (Denver, CO, USA).

Broth-microdilution assays were performed to determine the antibiotic susceptibility and minimum inhibitory concentrations (MICs) for 10 antibiotics, including ceftiofur, cefquinome, ceftazidime, ceftriaxone, meropenem, norfloxacin, ciprofloxacin, gentamycin, kanamycin and amikacin (Shanghai Macklin Biochemical Co., Ltd, China) as recommended by the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing. The above antibiotics are commonly used in veterinary clinical treatment of E. coli infection. Susceptibility to ceftiofur and cefquinome was determined in reference to previous research (23 (link)). All drugs were diluted 2-fold in MHB and mixed with an equal volume of bacterial suspension in a 96-well microtiter plate. Each test was repeated three times. Escherichia coli ATCC^® 25922™ was used as the quality-control strain (without magnolol). According to a previous report, bacteria resistant to ≥3 different classes of antibiotics were considered MDR (24 (link)).