Prevalence in the United States of aac(6')-Ib-cr encoding a ciprofloxacin-modifying enzyme.

Among 313 Enterobacteriaceae from the United States with a ciprofloxacin MIC of >/=0.25 microg/ml and reduced susceptibility to ceftazidime, aac(6')-Ib was present in 50.5% of isolates, and of these, 28% carried the cr variant responsible for low-level ciprofloxacin resistance. aac(6')-Ib-cr was geographically widespread, stable over time, most common in Escherichia coli, equally prevalent in ciprofloxacin-susceptible and -resistant strains, and not associated with qnr genes.