The Studies On The Mechanism Of Plasmid-mediated Quinolone Resistance Of Clinical Isolates In Enterobacteriaceae

Bacterial resistance to quinolones in Enterobacteriaceae has become anincreasing problem. Quinolones resistance in Enterobacteriaceae results mostly fromchromosomal mutations,but this resistance can not be horizontally transferred.Recently Robicsek first reported aac(6')-Ib-cr,a plasmid-associated gene of aaminoglycoside acetyltransferase.The gene of aac(6')-Ib-cr aquired unique ability ofmodifying quinolones because of two mutations at positions 223 and 454,but it onlycaused low-level resistance to quinolones which contained the amino nitrogen.Furthermore, aac(6')-Ib-cr can still modify some aminoglycosides,so it was capableof modifying two unrelated antimicrobial agents and reducing their activity. In thisstudy, we collected clinical strains from eight hospitals in Tianjin during 2001 and2002,study the mechanism of plasmid-mediated quinolone resistance of clinicalisolates in Enterobacteriaceae.PartⅠDetection of qnrA gene in quinolone-resistantclinical isolates in EnterobacteriaceaeObjective: To investigate the existence of qnrA gene among 116 ciprofloxacin-resistan clinical isolates in Enterobacteriaceae which were collected from eighthospitals in Tianjin.Methods: Agar dilution test was proformed to examin MIC of isolates to CPLX.The qnrA gene in ciprofloxacin-resistant isolates was examined with PCR.Results: 79 (68.1%) were highly resistant to ciprofloxacin (CPLX MIC≥32ug/ml) among 116 ciprofloxacin-resistant clinical isolates; None of the 116isolates harbored a qnrA gene.Conclusions: The qnrA gene was especially rare in Enterobacteriaceae inTianjin.Further study was in need to investigate epidemiology of qnr-like gene. PartⅡThe studies on the mechanism of quinoloneresistance by aac(6')-Ib-cr geneObjective: To investigate the existence of aac(6')-Ib gene in 77 ciprofloxacin-resistant E.coli isolates and study the mechanism of quinolone resistance byaac(6')-Ib-cr gene.Methods: The aac(6')-Ib gene in E.coli isolates was examined with PCR;Antimicrobial susceptibility test was proformed by agar dilution test;DNAsequencing was used for identification of aac(6')-Ib-cr; Conjugation experiment wasperformed to determine ways of transmission of resistance.Results. The gene of aac(6')-Ib was present in 15 E. coli isolates, in which therewere 10 isolates being resistant, 4 intermediate and 1 susceptible to kanamycin; DNAsequencing showed that 5 E. coli contained nucleotide changes at positions 223 (T→Cor T→A) and 454 (G→T), so they harbored aac(6')-Ib-cr gene; 4 isolates weresuccessful in conjugation experiments, and resistance to quinolones andaminoglycosides was partly transmitted to the recipent strain.Conclusions: This is the first report of aac(6')-Ib-cr gene from Tianjin;Itcontributes to resistance of quinolones and aminoglycosides in isolates to someextent.The gene of aac(6')-Ib-cr was located in a plasmid,so it raise risk factors ofhorizontal spread of resistance.