Enrofloxacin (ER) resistant Actinobacillus pleuropneumoniae strains emerged in Taiwan in 2002. The mechanism of ER resistance in A. pleuropneumoniae has not yet been reported. A total of 48 A. pleuropneumoniae isolates were obtained from the lungs of pigs with pleuropneumonia in Taiwan between September 2007 and April 2008. Twenty-nine isolates were found to be resistant to enrofloxacin. To understand the mechanisms of A. pleuropneumoniae's resistance to ER, enrofloxacin susceptibility of the isolates along with the mutations of the quinolone resistance-determining region (QRDR) of DNA gyrase and topoisomerase IV, qnr genes were analyzed. Enrofloxacin resistant isolates were found to carry at least one mutation in the QRDR of gyrA, leading to amino acid changes at codon 83 or 87. Efflux pump inhibitor (Phe-Arg-beta-naphthylamide) decreased enrofloxacin minimum inhibitory concentration 2-16-fold, suggesting participation of efflux in ER resistance. Plasmid mediated quinolone resistance genes qnr were not detected in these isolates. In conclusion, enrofloxacin resistance of A. pleuropneumoniae may be linked to multiple target gene mutations and active effluxs.
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