Emergence of plasmid-mediated quinolone resistance in Escherichia coli in Europe

Abstract

Although quinolone resistance results mostly from chromosomal mutations, it may also be mediated by a plasmid-encoded qnr gene in members of the family Enterobacteriaceae. Thus, 297 nalidixic-acid resistant strains of 2,700 Escherichia coli strains that had been isolated at the Bicetre Hospital (Le Kremlin-Bicetre, France) in 2003 were screened for qnr by PCR. A single E. coli isolate that carried a ca. 180-kb conjugative plasmid encoding a qnr determinant was identified. It conferred low-level resistance to quinolones and was associated with a chromosomal mutation in subunit A of the topoisomerase II gene. The qnr gene was located on a sul1-type class 1 integron just downstream of a conserved region (CR) element (CR1) comprising the Orf513 recombinase. Promoter sequences for qnr expression overlapped the extremity of CR1, indicating the role of CR1 in the expression of antibiotic resistance genes. This integron was different from other qnr-positive sul1-type integrons identified in American and Chinese enterobacterial isolates. In addition, plasmid pQR1 carried another class 1 integron that was identical to In53 from E. coli. The latter integron possessed a series of gene cassettes, including those coding for the extended-spectrum beta-lactamase VEB-1, the rifampin ADP ribosyltransferase ARR-2, and several aminoglycoside resistance markers. This is the first report of plasmid-mediated quinolone resistance in Europe associated with an unknown level of plasmid-mediated multidrug resistance in Enterobacteriaceae.

FIG. 1.

Plasmid DNAs from clinical isolate E. coli Lo and E. coli transconjugant strains (A) and Southern hybridization of plasmid DNAs with the qnr-specific probe (B) and the bla_VEB-1-specific probe (C). Lanes: 1, E. coli Lo; 2, E. coli J53/pQR1; 3, E. coli J53/pMG252 (used as a positive control); M, E. coli NCTC50192 (used as a negative control and a reference for plasmid sizes).

FIG. 2.

Comparison of sul1-type integrons that contain a qnr gene (A) and the bla_VEB-1-positive class 1 integron In53 (B). Shaded boxes in In53 indicate gene cassettes possessing their own promoter sequences.

FIG. 3.

Nucleotide sequence of the promoter structure for qnr expression, as determined by the 5′ RACE experiment. The deduced amino acid sequence of Qnr is designated in single-letter code below the nucleotide sequence. The transcription orientation of the qnr gene is indicated by a horizontal arrow. The right-hand boundary of CR1 is shaded in gray. The −35 and −10 promoter sequences are boxed, as is the +1 transcription initiation site; all of these are part of the CR1 element. The vertical arrow indicates the position of the extremity of the CR1 right-hand boundary of In36 (24), in which the 67-bp sequence with a dotted underlined is lacking.