Detection and genetic characterisation of qnrB in hospital isolates of Klebsiella pneumoniae in Singapore

Polymerase chain reaction (PCR) screening of 116 ciprofloxacin-resistant Klebsiella pneumoniae hospital isolates for the presence of qnr genes that mediate plasmid quinolone resistance revealed that none were positive for qnrA or qnrS. However, qnrB was detected in ca. 5.2% of the isolates. Southern hybridisation demonstrated that the qnrB-hybridising plasmids were large (>70 kb) and capable of transferring quinolone resistance by conjugation. Sequence analysis of the qnrB genes detected in this study showed that they were identical to previously identified qnrB1, qnrB4 and qnrB6 genes, although a novel variant designated qnrB20 was also identified. Analysis of the genetic environment around the cloned qnrB genes showed that they were present in diverse plasmid backbones, sometimes within novel genetic contexts, but always associated with mobile or transposable elements.