Molecular characterisation of resistance to fluoroquinolones in Haemophilus parasuis isolated from China.

There are many studies on fluoroquinolone (FQ) resistance in the literature, but little is known regarding the molecular characterisation of FQ-resistant Haemophilus parasuis. In this study, a total of 138 H. parasuis isolates were examined, among which 83 (60.1%) were resistant to enrofloxacin (EFX) and 8 (5.8%) were resistant to levofloxacin (LFX) as determined by Etest. Ten point mutations in the quinolone resistance-determining regions of gyrA, gyrB, parC and parE were detected by PCR and DNA sequencing. Interestingly, 100% of the resistant isolates contained mutations at 87D of gyrA, but other mutations occurred less frequently. Furthermore, it was found that there was synergy between 73S→R/I in parC and other point mutations with respect to FQ resistance. To examine the effect of different point mutations on FQ resistance, the minimum inhibitory concentrations of EFX and LFX were determined for strains generated by site-directed mutagenesis, and three point mutations (gyrA 87D→N, parC 73S→R and parE 551T→A) were shown to be involved in FQ resistance.