Analysis of plasmid-mediated multidrug resistance in Escherichia coli and Klebsiella oxytoca isolates from clinical specimens in Japan

This study investigated the relationship of plasmid-mediated quinolone resistance (PMQR) and aminoglycoside resistance among oxyimino-cephalosporin-resistant Escherichia coli (n = 46) and Klebsiella oxytoca (n = 28) clinical isolates in Japan. Seventy-three isolates appeared to produce an extended-spectrum β-lactamase (ESBL) and one K. oxytoca isolate produced IMP-1 metallo-β-lactamase (MBL). Polymerase chain reaction (PCR) and sequencing confirmed that eight CTX-M-9/SHV-12-producing isolates, one IMP-1-producing K. oxytoca isolate, and six ESBL-positive E. coli isolates respectively possessed PMQR genes qnrA1, qnrB6, and aac(6′)-Ib-cr. All qnr-positive isolates also carried either aac(6′)-Ib or aac(6′)-IIc aminoglycoside acetyltransferase genes. Resistance determinants to β-lactams, quinolones and aminoglycosides were co-transferred with a plasmid of ca. 140 kb. The qnrA1 gene was located downstream of insertion sequence ISCR1 in complex class 1 integrons. A novel qnrA1-carrying class 1 integron with the cassette arrangement aac(6′)-IIc–aadA2 as well as a unique class 1 integron with blaIMP-1–aac(6′)-IIc cassettes on the plasmid carrying qnrB6 were found in K. oxytoca isolates. We describe the identification of qnrB6 and aac(6′)-Ib-cr and the close association of qnr with aac(6′)-Ib and aac(6′)-IIc for the first time in clinical isolates producing ESBL or MBL in Japan.