National Institute of Animal Health (NIAH)

Topics in Animal Health Research 2009

21. Characterization of fluoroquinolone-resistant Mannheimia haemolytica isolates from cattle with bovine pneumonia


  This study was conducted to characterize quinolone-resistant isolates of Mannheimia haemolytica. We determined the antimicrobial susceptibility of 407 M. haemolytica isolates, which were obtained from cattle with bovine respiratory disease between 1984 and 2009. The resistance rates for nalidixic acid, enrofloxacin, and danofloxacin were 32.7%, 9.6%, and 9.6%, respectively. The nucleotide sequences of the quinolone resistance-determining regions of the gyrA and parC genes of nalidixic acidresistant M. haemolytica were determined. All nalidixic acid-resistant strains possessed at least 1 amino acid substitution in each of the GyrA and ParC fragments investigated. These results suggest that M. haemolytica requires at least 1 amino acid substitution in both GyrA and ParC in order to attain significant levels of resistance to quinolones. All fluoroquinolone-resistant isolates belonged to serotype 6, and pulsed-field gel electrophoresis (PFGE) analysis revealed that their genotype was identical. These results indicate that fluoroquinolone-resistant M. haemolytica strains have clonally expanded.
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Katsuda, K. et al. (2009) Vet. Microbiol. 139(1-2):74-79.