The β-Lactamase Disk Test: A Modified Method to Detect Extended-Spectrum-β-Lactamases in Multidrug-Resistant Escherichia coli Isolates

Most urinary tract infections (UTIs) are caused by Escherichia coli (E. coli) species. Due to infections outbreaks of E. coli strains with multiple mechanisms of resistance to β-lactam antibiotics, the sensitivity of confirmatory tests to detect the extended spectrum β-lactamases (ESBLs) have decreased. The current study aimed to introduce a modified method to detect ESBLs in Gram-negative bacilli. Totally, 86 clinical isolates of E. coli resistant to extended-spectrum cephalosporins were collected from patients with UTIs in Kerman, Iran. The susceptibility to antibiotics was determined by disk diffusion method. ESBLs producing isolates were identified by combination double disk synergy test (CDDST) and β-lactamase disk test. The β-lactamase genes including blaTEM, blaSHV, blaCTX-M, blaOXA-1 and blaPER were detected by polymerase chain reaction (PCR) method and sequenced. All of the isolates were multidrug-resistant (MDR). In the current study 88% and 97.6% of the isolates were considered as ESBLs producing by CDDST and β-lactamase disk test, respectively. At least 92% of the isolates were positive for one of the blaCTX-M, blaTEM, blaOXA-1 and blaSHV genes. The blaCTX-M, blaTEM, blaOXA-1 and blaSHV genes were detected in 74.4%, 61.6%, 14% and 2.3% of the isolates, respectively. The β-lactamase disk test is appropriately sensitive to detect ESBLs in MDR isolates of E. coli.