Plasmidic qnr Genes Confer Clinical Resistance to Ciprofloxacin under Urinary Tract Physiological Conditions

Escherichia coli variants expressing plasmid-mediatedqnr genes are usually susceptible to fluoroquinolones by standard susceptibility testing. Here we show that, under specific urinary tract physiological conditions, susceptible laboratory and clinical strains harboringqnr determinants become fully resistant to ciprofloxacin (CIP). Therefore, physiological conditions, mainly urine pH values, should be considered when performing susceptibility testing of CIP activity againstE. coli in treating urinary tract infection (UTI) and for selecting appropriate antibiotics for UTI treatment.