| As quinolones used widely in clinical medicine, the increasing prevalence of resistantisolates among Escherichia coli has been an emerging problem. Previous studiesdemonstrated that the mechanisms of resistance to quinolones in E. coli are mainlychromosome-mediated, alterations in target genes and efflux pumps. Plasmid-mediatedquinolone resistance （PMQR） was found to be the primary route of transmission of resistancesubsequently. Currently, studies related to resistance to quinolones among foodborne isolatesof E. coli, however, are limited and primarily focused on clinical isolates. In this study, E. coliisolates from food and clinical patients were determined their susceptibility to14antimicrobial agents, the presence of class I integrons and the mechanisms of resistance toquinolones in E. coli. The objectives were to understand the prevalence and horizontaltransfer of resistance genes in E. coli isolates from different sources and find out the effectivemeasures to control the dissemination of bacterial resistance. The results were as follows:（1） A total of33E. coli isolated from cooked meat products and66clinical isolates weresubjected to antimicrobial susceptibility testing. Resistance to sulfamethoxazole,trimethoprim/sulfamethoxazole, ampicillin, tetracycline, and streptomycin were observedmost often, whilst amikacin and chloramphenicol exhibited good activity against theseisolates. Eighteen foodborne isolates （54.5%） were multiresistance, expressing resistance tothree or more antimicrobials. Multidrug resistance （MDR） was common in clinical isolates.All isolates were resistance to at least three antimicrobials; nearly50.0%of the isolates wereresistance to at least ten antimicrobials; two isolates were resistance to all14antimicrobialsused in this study.（2） Class I integrons were detected in18.2%of foodborne isolates and54.5%of clinicalisolates. Integron-positive isolates from food contained three groups of resistance genecassette, consisting of dfrA17-aadA5, dfrA1-aadA1, and dfrA12-orfF-aadA2. Clinical isolatescarried integrons contained seven groups of resistance gene cassette, consisting ofdfrA17-aadA5, dfrA12-orfF-aadA2, dfr2d, aacA4-catB8-aadA1, aadA2+dfrA12-orfF-aadA2,dfrA5, and dfr2d+dfrA17-aadA5.（3） qnr genes were absent in foodborne isolates. Among clinical isolates, qnr genes were detected in7isolates （2isolates carried qnrA1and5isolates carried qnrS1）; aac（6’）-Ib genewas detected in20isolates and eleven carried aac（6’）-Ib-cr gene. Quinolone resistance couldbe transferred to recipient E. coli J53by conjugation from five qnr-positive isolates, with thetransfer frequency in the range of3.4×10-5to2.4×10-3. The donors showed higher levels ofresistance to quinolones relative to their transconjugants. DNA sequencing of the PCRproducts covering the entire quinolone-resistance determining regions （QRDRs）demonstrated that mutations in GyrA and/or ParC were present in all donor isolates and therewere no mutations in the target genes among the transconjugants and the recipient. It wassuggested that chromosomal mutations in QRDRs played an important role in mediatinghigh-level quinolones resistance and the presence of qnr alone conferred a low level ofquinolones resistance. Transconjugants showed32-to128-fold and16-to32-fold increasesin the minimal inhibitory concentrations （MICs） of ciprofloxacin and levofloxacinrespectively, and exhibited higher levels of resistance to gentamicin, amikacin, ceftazidime,cefoperazone, cefepime, ampicillin, and sulfamethoxazole relative to recipient.（4） The levels of qnrS-specific transcripts were compared using real-time quantitativePCR. Three transconjugants obtained in the present study had different ciprofloxacin MICs,however, the results showed no significant differences in the basal expression levels of qnrSgene. The qnrS transcript levels increased when ciprofloxacin and levofloxacin was present,suggesting that the qnrS expression was associated with quinolone resistance.（5） Five qnr-positive isolates were subjected to plasmid curing by variabletemperature-SDS method and three isolates obtained eliminators successfully with the curingfrequency of60.4%to70.8%. After curing, there were obvious changes in the plasmidpatterns of the three isolates, and qnr genes were not detected in these isolates by PCR. Theresistance profiles of the isolates changed after curing. Before curing, the isolates wereresistant to gentamicin, amikacin, ceftazidime, cefoperazone, cefepime, ampicillin, andsulfamethoxazole; after curing, the isolates become susceptible to these antimicrobials.Nevertheless, no changes were observed in the two isolates which failed to plasmid curing. |
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