Citrobacter spp. as a Source ofqnrBAlleles

Abstract

ABSTRACTqnrBis the most common of the fiveqnrfamilies and has the greatest number of allelic variants. Almost two-thirds of theqnrBalleles have been reported inCitrobacterspp., and several were shown to be located on the chromosome. In this study, PCR was used to investigate the prevalence of plasmid-mediated quinolone resistance genes in 71 clinical isolates belonging to theCitrobacter freundiicomplex. Thirty-seven percent containedqnrBalleles, including 7 (qnrB32 to qnrB38) that were novel and 1 pseudogene, while none containedqnrA,qnrC,qnrD,qnrS, oraac(6′)-Ib-cr. When the strains were arrayed by related 16S rRNA sequence and further separated into subspecies by biochemical criteria, clustering ofqnrB-positive strains was evident. In only two strains withqnrB2andqnrB4was quinolone resistance transferable by conjugation, and only these strains contained the ISCR1sequence that is often associated withqnrBon plasmids. Five of 26qnrB-positive strains contained integrase genes, but these included the strains withqnrB2andqnrB4as well as two strains with other transmissible plasmids. In a fully sequenced genome ofCitrobacter youngae, a member of theC. freundiicomplex, another novelqnrBallele,qnrB39, occurs in a sequence of genes that is 90% identical to sequence surrounding integron-associatedqnrB4incorporated into plasmids. The chromosome ofCitrobacteris the likely source of plasmid-mediatedqnrB.