Antibiotic Resistance Mechanisms of Escherichia coli Isolates from Urinary Specimens

Background : This study was designed to characterize urinary isolates of Escherichia coli that produce extended-spectrum beta-lactamases (ESBLs) and to determine the prevalence of other antimicrobial resistance genes. Methods: A total of 264 non-duplicate clinical isolates of E coli were recovered from urine specimens in a tertiary-care hospital in Busan in 2005. Antimicrobial susceptibility was determined by disk diffusion and agar dilution methods, ESBL production was confirmed using the double-disk synergy (DDS) test, and antimicrobial resistance genes were detected by direct sequencing of PCR amplification products. E coli isolates were classified into four phylogenetic biotypes according to the presence of chuA, yjaA, and TSPE4. Results : DDS testing detected ESBLs in 27 (10.2%) of the 264 isolates. The most common type of ESBL was CTX-M-15 (N=14), followed by CTX-M-3 (N=8) and CTX-M-14 (N=6). All of the ESBL-producing isolates were resistant to ciprofloxacin. PCR experiments detected genes encoding DHA-1 and CMY-10 AmpC beta-lactamases in one and two isolates, respectively. Also isolated were 5 isolates harboring 16S rRNA methylases, 2 isolates harboring Qnr, and 19 isolates harboring AAC(6')-lb-cr. Most ESBL-producing isolates clustered within phylogenetic groups B2 (N=14) and D (N=7). Conclusion : CTX-M enzymes were the dominant type of ESBLs in urinary isolates of E coli, and ESBL-producing isolates frequently contained other antimicrobial resistance genes. More than half of the urinary E coli isolates harboring CTX-M enzymes were within the phylogenetic group B2. (Korean J Inb Med 2009;29-17-24)