qnr Prevalence in Ceftazidime-Resistant Enterobacteriaceae Isolates from the United States

We screened 313 ceftazidime-resistant Enterobacteriaceae isolates obtained in the United States from 1999 to 2004 for all three known qnr genes. A qnr gene was present in 20% of Klebsiella pneumoniae isolates, 31% of Enterobacter sp. isolates, and 4% of Escherichia coli isolates. qnrA and qnrB occurred with equivalent frequencies and, except for qnrB in enterobacters, were stable over time. qnrS was absent.