Abstract: Using ciprofloxacin-resistant strains of Listeria monocytogenes（LM）isolated from food as research object,this study aims to understand the quinolone-resistant mechanisms of LM. The minimum inhibitory concentration（MIC）of ciprofloxacin by the strain was determined by the agar dilution method. The mutations in the quinolone resistance-determining region（QRDR）and the presence of plasmid-mediated quinolone resistance（PMQR）genes were investigated using PCR. Expression levels of lde gene under different conditions were detected by qRT-PCR. To address the role of efflux pump Lde in ciprofloxacin resistance,deletion mutant in lde was constructed using SOE-PCR. Results showed that the amino acid sequences of GyrA,GyrB,ParC,and ParE in 15 resistant strains showed 100% similarity with the sequences of sensitive strains,and no PMQR genes were detected in any of the resistant strains. In the presence of reserpine,an efflux pump inhibitor,the MICs of ciprofloxacin by strain L28 and L47 reduced to the 1/8 and 1/4 of original one. The expression of lde was observed in all sensitive and resistant strains by similar values. After treatment with ciprofloxacin,the resistant strains showed significant increases in lde,however,no obvious changes in expression of lde were observed in the sensitive strains. In the presence of reserpine,the expression of lde significantly inhibited in resistant strains. The mutant lacking the lde gene was susceptible to ciprofloxacin and its MIC of ciprofloxacin did not change in the presence of reserpine. Our data demonstrated that efflux pump Lde mediated quinolone resistance in L. monocytogenes.

Key words: Listeria monocytogenes, quinolone, antimicrobial resistance, efflux pump