Microbial Drug Resistance vol:13 issue:3 pages:166-170
The quinolone resistance-determining regions (QRDR) of gyrA, gyrB, parC, and parE of ten Mycoplasma
hyopneumoniae field isolates that were either sensitive (5) or resistant (5) to the fluoroquinolones flumequine
and enrofloxacin were characterized. In all five resistant isolates, one point mutation (C A) in parC was
found, resulting in an amino acid change from serine to tyrosine at position 80 (Escherichia coli numbering).
For four of these isolates, this was the only mutation found. These isolates had a minimum inhibitory concentration
(MIC) of enrofloxacin of 0.5 g/ml, whereas for sensitive isolates the MIC of enrofloxacin was
0.06 g/ml. One resistant isolate (Mh 20) had an extra mutation (C T) in gyrA resulting in an amino acid
change from alanine to valine at position 83 (E. coli numbering), leading to a further increase in the MIC of
enrofloxacin (1 g/ml). No mutations resulting in an amino acid change were detected in the QRDR of the
gyrB and parE genes of the selected isolates. This is the first description of the mechanism of stepwise resistance
against fluoroquinolones in M. hyopneumoniae.