Drug Susceptibility Test and Analysis of Quinolone Resistance Genes in Mycoplasma hyopneumoniae Vaccine Strains and Field Isolates from China

Background: Enrofloxacin is a commonly used animal-specific drug in veterinary clinics. However, this drug has no epidemiological cutoff values (ECVs/ECOFFs) for Mycoplasma hyopneumoniae in CLSI and EUCAST. Defining the epidemiological cutoff values (ECOFFs) of enrofloxacine to Mycoplasma hyopneumoniae (M. hyo) can inform an early detection of bacterial resistance to better manage the resistance prevention and also help in establishing drug resistance breakpoints;.Methods: We determined the susceptibility breakpoint ofM. hyo to enrofloxacin by the American Clinical and Laboratory Standards Institute (CLSI) standard method based on the PCR of vaccine strains and wild strains drug resistance genes;.Results: Eighty strains ofM.hyo isolated in Tibet were moderately sensitive (S) to tetracycline, florfenicol, spiramycin, erythromycin thiocyanate, tilmicosin, tiamulin, lincomycin, clindamycin, ofloxacin, enrofloxacin, gentamicin, amikacin, with MICs below 0.5 mu g/ mL. For vaccine 168L, RM48, and J strains, the susceptibility to the same antibacterial drugs was lower compared to the Tibetan isolates. The resistance of J strain to erythromycin thiocyanate was confirmed. Gene point mutation was confirmed in Quinolone Resistance Determining Regions (QRDR) of HNSH strain Topoisomerase IV subunit A, this finding is compared with the sequencing results of 168L strain reference sequence (Accession number: CP003131). Arg-Lys amino acid mutation (G921A and G1179A) was confirmed for the increase of MIC value involved in M.hyo to enrofloxacine;.Conclusion: The cut-off value of M.hyo to enrofloxacin was set as 1 mu g/mLby ECOFFinder XL 2010 V2.1.