Objective: To investigate the mechanism of the different levels of ciprofloxacin resistance in qnrA-containing transconjugants. Methods: E.coli J53Az R as the recipient, 4 qnrA -containing transconjugants were constructed by conjugation from 4 qnrA -carrying clinical isolates. MICs of the transconjugants were measured by E test. aac(6')-Ib-cr was detected by PCR, and qnrA mRNA expression level was determined by real-time RT-PCR. The promoter sequences of qnrA were amplified by PCR from qnrA-bearing plasmids and cloned into plasmid pKK232-8, then transformed into HB101. All promoter fragments were sequenced. Results: The MICs of ciprofloxacin against 4 transconjugants demonstrated a 10-fold difference from 0.094 μg/ml to 1.000 μg/ml. Of 4 qnrA-bearing plasmids in E. coli J53, ciprofloxacin MICs of pHS4 and pHS5 were 0.094 μg/ml and 0.125 μg/ml, respectively; pHS3, which contained the aac(6')-Ib-cr gene as well, MIC was 0.25 μg/ml; and pHS6, which had a high expression level of qnrA and the aac(6')-Ib-cr gene, MIC was 1.00 μg/ml. The relative expression levels of qnrA mRNA in J53 pHS6 was 32.5, much higher than the other 3 transconjugants (from 1.0 to 2.5). The promoter in plasmid pHS6 was 12-fold stronger than that in the other 3 plasmids. Compared with pHS3, there was 7 bp (GTTAGCA) deletion between the transcription initiation site and the start of qnrA in pHS6. Conclusion: Co-existence of qnrA and aac(6') -Ib-cr in a single plasmid and high level of qnrA expression can account for the different levels of ciprofloxacin resistance in transconjugants.
|Original language||English (US)|
|Number of pages||5|
|Journal||Chinese Journal of Microbiology and Immunology|
|State||Published - Dec 1 2008|
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