Mechanisms of reduced susceptibility to ciprofloxacin in Escherichia coli isolates from Canadian hospitals

Abstract

OBJECTIVE: To determine whether plasmid-mediated quinolone resistance (PMQR) determinants play a role in the increasing resistance to fluoroquinolones among Escherichia coli isolates in Canadian hospitals, and to determine the mechanisms of reduced susceptibility to ciprofloxacin in a recent collection of 190 clinical E coli isolates. METHODS: E coli isolates (n=1702) were collected as part of the 2007 Canadian Hospital Ward Antibiotic Resistance Surveillance (CANWARD) study. Antimicrobial susceptibility testing was performed by Clinical and Laboratory Standards Institute (CLSI) broth microdilution. Using a representative subset of isolates (n=190), the mechanisms of reduced susceptibility to ciprofloxacin were detected by polymerase chain reaction and sequencing of the quinolone resistancedetermining regions (QRDR) of chromosomal gyrA and parC genes, and by polymerase chain reaction for the PMQR genes: qnr, aac(6) Ib-cr and qepA. RESULTS: 2.1% and 1.1% of E coli harboured aac(6)Ib-cr and qnrB, respectively. Single amino acid substitutions in the QRDR of gyrA were observed among isolates with ciprofloxacin minimum inhibitory concentrations as low as 0.12 μ/mL. As the ciprofloxacin minimum inhibitory concentration increased to 1 μ/mL (which is still considered to be susceptible by the CLSI), the vast majority of isolates demonstrated both gyrA and parC mutations. Conclusion: PMQR determinants and QRDR mutants among clinical E coli isolates with reduced susceptibility to ciprofloxacin demonstrates the need for increased surveillance and the need to re-evaluate the current CLSI breakpoints to prevent further development of fluoroquinolone resistance. ©2012 Pulsus Group Inc. All rights reserved.