Temporal appearance of plasmid-mediated quinolone resistance genes.

One hundred fifty AAC(6')-Ib-positive gram-negative isolates collected between 1981 and 1991 were examined by PCR for the presence of the aac(6')-Ib-cr variant and other plasmid-mediated quinolone resistance (PMQR) genes. None had the aac(6')-Ib-cr variant, qnrA, qnrS, qnrC, or qepA, but two strains collected in 1988 had qnrB alleles, making these the earliest known PMQR genes.