2019 Volume 17 Issue 4 Pages 262-272
Wastewater treatment plays an important role in controlling the release of antimicrobial resistant bacteria to the environment. We characterized extended spectrum β-lactamase (ESBL)-producing Escherichia coli strains isolated from the environment by cefotaxime (CTX)-containing agar plates in this study. The results on the samples under the influence of treated wastewater showed that 1.2% to 5.3% of the total E. coli population formed colonies on the ECC medium containing 4 µg/mL CTX and all of them were ESBL-producing strains. Addition of 4 µg/mL CTX to the ECC medium imposed a selection pressure equivalent to MIC higher than 64 µg/mL (MIC breakpoint in the previous Clinical and Laboratory Standards Institute [CLSI] criteria) because the medium containing CTX not only inhibited the growth of the strains with lower MICs than the concentration in the medium but also suppressed the formation of colonies of the strains with higher MICs. Adding 64 µg/mL CTX in the ECC medium selected only clinically important strains with an intensively resistant spectrum covering cephalosporins (including ceftazidime) and fluoroquinolones. Some isolates harbored multiple ESBL-producing genes. The dominant genes encoding ESBL were blaCTX-M group 1, blaCTX-M group 9, and blaTEM group.