New Plasmid-Mediated Quinolone Resistance Gene, qnrC, Found in a Clinical Isolate of Proteus mirabilis▿

Since the discovery of qnrA in 1998, two additional qnr genes, qnrB and qnrS , have been described. These three plasmid-mediated genes contribute to quinolone resistance in gram-negative pathogens worldwide. A clinical strain of Proteus mirabilis was isolated from an outpatient with a urinary tract infection and was susceptible to most antimicrobials but resistant to ampicillin, sulfamethoxazole, and trimethoprim. Plasmid pHS10, harbored by this strain, was transferred to azide-resistant Escherichia coli J53 by conjugation. A transconjugant with pHS10 had low-level quinolone resistance but was negative by PCR for the known qnr genes, aac(6′)-Ib-cr and qepA . The ciprofloxacin MIC for the clinical strain and a J53/pHS10 transconjugant was 0.25 μg/ml, representing an increase of 32-fold relative to that for the recipient, J53. The plasmid was digested with HindIII, and a 4.4-kb DNA fragment containing the new gene was cloned into pUC18 and transformed into E. coli TOP10. Sequencing showed that the responsible 666-bp gene, designated qnrC , encoded a 221-amino-acid protein, QnrC, which shared 64%, 42%, 59%, and 43% amino acid identity with QnrA1, QnrB1, QnrS1, and QnrD, respectively. Upstream of qnrC there existed a new IS 3 family insertion sequence, IS Pmi1 , which encoded a frameshifted transposase. qnrC could not be detected by PCR, however, in 2,020 strains of Enterobacteriaceae . A new quinolone resistance gene, qnrC , was thus characterized from plasmid pHS10 carried by a clinical isolate of P . mirabilis .