Minimum Inhibitory Concentration and Antimicrobial Susceptibility Test

The antibiotic resistance of 59 ESBL/AmpC producing E. coli isolates was evaluated by minimum inhibitory concentration (MIC) of cefotaxime and antimicrobial susceptibility test (AST). MIC testing was performed using a micro-broth dilution method that followed the Clinical and Laboratory Standards Institute (CLSI) guidelines (CLSI, 2015, 2016). The concentration of cefotaxime was serially diluted with Mueller Hinton Broth (MHB) from 0 to 256 μg/mL with the MIC defined by the highest concentration among three replicates. The ESBL-producing strain KCJ1409 was used as a positive control, and DH5α as a negative control (Mir et al., 2016). The standard Kirby Bauer disk diffusion method on Mueller Hinton Agar (MHA) was used to test susceptibility against the following 13 antibiotics: amikacin (30 μg), ampicillin (10 μg), amoxicillin/clavulanic acid (30 μg), sulfisoxazole (250 μg), ceftiofur (30 μg), chloramphenicol (30 μg), cephalothin (30 μg), gentamicin (10 μg), nalidixic acid (30 μg), streptomycin (10 μg), sulfamethoxazole/trimethoprim (23.75 μg/1.25 μg), tetracycline (30 μg), and colistin (10 μg) (BD, United States). E. coli (ATCC 35401), Staphylococcus aureus (ATCC 25923), and Pseudomonas aeruginosa (ATCC 27853) were used as controls.