Prevalence of plasmid-mediated quinolone resistance determinants QnrA, QnrB, and QnrS among clinical isolates of Enterobacter cloacae in a Taiwanese hospital

Abstract

The prevalence of three plasmid-mediated quinolone resistance determinants, QnrA, QnrB, and QnrS, among 526 nonreplicate clinical isolates of Enterobacter cloacae collected at a Taiwanese university hospital in 2004 was determined by PCR and colony hybridization, and the association of Qnr with the IMP-8 metallo-beta-lactamase was investigated. Eighty-six (16.3%) of all isolates were qnr positive, and the qnrA1-like, qnrB2-like, and qnrS1-like genes were detected alone or in combination in 3 (0.6%), 53 (10.1%), and 34 (6.5%) isolates, respectively. Among 149 putative extended-spectrum-beta-lactamase-producing isolates, 59 (39.6%) isolates, all of which were SHV-12 producers, harbored qnrA (0.7%; 1 isolate), qnrB (28.9%; 43 isolates), or qnrS (12.1%; 18 isolates). Forty-four (78.6%) of 56 IMP-8 producers carried qnrB (58.9%; 33 isolates), qnrS (25.0%; 14 isolates), or both. PCR and sequence analysis revealed that qnrA1 was located in a complex sul1-type integron that contains dhr15, aadA2, qacEDelta1, sul1, orf513, qnrA1, ampR, and qacEDelta1. Conjugation experiments revealed the coexistence of qnrB and bla(IMP-8) on the transferred plasmids and the absence of beta-lactamase content on the transferred qnrS-positive plasmids. The transferred bla(IMP-8)-positive plasmids with and without qnrB had very similar restriction patterns, suggesting the horizontal mobility of qnrB. Pulsed-field gel electrophoresis showed six major patterns among the 44 qnr-positive IMP-8-producing isolates. Thus, the extremely high prevalence of qnr among the metallo-beta-lactamase-producing E. cloacae isolates in the hospital may be due mainly to the intrahospital spread of a few clones and the dissemination of plasmids containing both qnrB and blaIMP-8.

FIG. 1.

PFGE patterns of 44 qnr-positive IMP-8-producing E. cloacae isolates. Lane M, lambda ladder. The numbers of isolates that were represented by each pattern are shown below the gel.

FIG. 2.

EcoRI restriction patterns of conjugative plasmids. The results of Southern hybridization with the digoxigenin-labeled qnr probes are shown below the gels. Asterisks indicate the hybridized restriction fragments. Lane M1, molecular marker II (Roche Applied Science); lane M2, 1-kb molecular marker (Promega Co., Taipei, Taiwan, Republic of China). (A) Transferred qnrS-positive plasmids and hybridization with the qnrS probe. Lanes 1 and 2, restriction pattern A. (B) Transferred bla_IMP-8-positive plasmids that were positive (lanes 1 to 5) and negative (lanes 6 to 12) for qnrB and hybridization with the qnrB probe. Lanes 1 to 12, restriction patterns B1 to B12.