Objectives: To analyse the in vitro transfer of the qnrA1 gene by a 50 kb (pSZ50) self-transferable plasmid that derives from a 300 kb plasmid (pSZ300) and to determine the complete nucleotide sequence of plasmid pSZ50.
Methods: Extended-spectrum β-lactamase (ESBL) and plasmid-mediated quinolone resistance (PMQR) genes of an Escherichia coli clinical isolate were analysed. Plasmid analysis included conjugation and selection on seven antibiotics examined by antimicrobial susceptibility testing, RFLP comparison, Southern hybridization, incompatibility group identification and shotgun sequencing.
Results: The E. coli 5509 isolate carries the genes encoding the ESBL CTX-M-15 and the quinolone resistance determinants qnrA1, qnrB2 and aac(6')-Ib-cr on a 300 kb plasmid. Seven transfer resistances were analysed by conjugation under two conditions (30 and 37°C), leading to two distinct transconjugant phenotypes with different resistances. Transconjugants of phenotype A harboured a 300 kb plasmid named pSZ300 that conferred resistance to eight antibiotics and harboured the qnrA1, aac(6')-Ib-cr and bla(CTX-M-15) genes. Transconjugants of phenotype B were resistant to three antibiotics and they harboured the qnrA1 gene on an ≈ 50 kb plasmid named pSZ50. Both plasmids were self-transferable at a frequency of 1 × 10(-3). Plasmid pSZ300 was typed to be both an IncF and IncN plasmid, whereas pSZ50 corresponded only to type IncN. Fingerprinting and Southern hybridization showed that plasmid pSZ50 derived from pSZ300. The complete nucleotide sequence of plasmid pSZ50 was determined (51556 bp) and 55 open reading frames were predicted. The qnrA1 gene was identified in a tandem duplicate inside a sul1-type integron structure.
Conclusions: The plasmid pSZ300 represented a fusion of two replicons (IncF and IncN), and our observations suggest that the plasmid pSZ50 (IncN) may split and transfer antibiotic resistance determinants. This mechanism could be advantageous in the dissemination of antibiotic resistance genes.