Detection of ESBL- and AmpC-producing E. coli isolates from urinary tract infections

Extended-spectrum β-lactamases (ESBLs) and AmpC enzymes have been observed in virtually all species of the family Enterobacteriaceae. The β-lactamase producing bacteria cause many serious infections, including urinary tract infections. These enzymes are predominantly plasmid mediated. There are no recommended guidelines for detection of this resistance mechanism and there is a need to address this issue as much as the detection of ESBLs. This study was undertaken to characterize ESBL and AmpC producers among Escherichia coli by polymerase chain reaction (PCR), which were initially screened by phenotypic method.

A total of 90 isolates of E. coli were recovered from the urinary tract during a 7-month period, and were screened for ESBLs and AmpC production by disk diffusion test using cefoxitin (30 μg) disks and confirmed by combined disk diffusion test using phenyl boronic acid. The presence of genes encoding CIT, FOX, and TEM was detected by PCR.

On disk diffusion test, 59 of 90 isolates were resistant to third generation of cephalosporins; of these 37 (62.7%) and 3 (5%) were ESBL and AmpC producers, respectively. PCR showed that 29 (49.1%) and 3 (5%) were positive for blaTEM and blaCMY-2 , respectively.

ESBL- and AmpC-producing E. coli isolates cause significant resistance to cephalosporin. There is a need for a correct and reliable phenotypic test to identify AmpC β-lactamases and to discriminate between AmpC and ESBL producers. This work showed that boronic acid can differentiate ESBL enzymes from AmpC enzymes.