Characterization of chromosomal qnrB and ampC alleles in Citrobacter freundii isolates from different origins.

The association of ESBLs (extended-spectrum beta-lactamases)/pAmpCs (plasmid-mediated AmpC β-lactamases) with PMQR (plasmid mediated quinolone resistance) in gram-negative bacteria has been of great concern. The present study was performed to characterize the diversity, gene location, genetic context, and evolution of ampC and qnrB alleles in isolates of Citrobacter freundii. Fifteen isolates of C. freundii were identified from a total of 788 isolates of Enterobacteriaceae derived from humans, animals, animal food products, and the environment between 2010 and 2012. Co-existence of qnrB/ΔqnrB with ampC was detected in all C. freundii isolates. Both ampC and qnrB genes were found to be located on the chromosome, but were distantly separated on the chromosome. Seven and six novel alleles were discovered for the 10 ampC and qnrB variants detected in this study, respectively. Phylogenetic analysis showed that the new alleles differed a little from the variants of ampC/qnrB previously described in this genus. The genetic context surrounding ampC genes was AmpR-AmpC-Blc-SugE. However, five different genetic contexts surrounding qnrB/ΔqnrB genes were observed, but they occurred in all cases between the pspF and sapA genes. Additionally, cloning experiments showed that the regions containing different qnrB alleles, even with different genetic contexts, contributed to the reduction of quinolone susceptibility. Our results showed that the chromosomal ampC and qnrB alleles are closely related to C. freundii. However, unlike ampC, qnrB alleles seemed to be related to the genetic contexts surrounding them. The evolution of these two genes in C. freundii isolates might be through different pathways.