Antimicrobial resistance and genetic characterization of fluoroquinolone-resistant Mannheimia haemolytica isolates from cattle with bovine pneumonia.

Antimicrobial susceptibility and molecular characterization of quinolone-resistant Mannheimia haemolytica was conducted. The antimicrobial susceptibility of 229 M. haemolytica isolates which were obtained from cattle with bovine respiratory disease during the period 1984-2006, was determined using 14 antimicrobial agents. Of the 229 isolates, 114 (49.8%) were resistant to at least one agent and resistance rates ranged from 4.8% to 31.4%. Resistance rates for dihydrostreptomycin, oxytetracycline, doxycycline, ampicillin, amoxicillin, thiamphenicol, kanamycin chloramphenicol, nalidixic acid, enrofloxacin, and danofloxacin were 31.4%, 20.5%, 18.3%, 19.2%, 16.6%, 10.9%, 11.4%, 10.5%, 17.0%, 4.8% and 4.8%, respectively. The nucleotide sequences of the quinolone resistance-determining regions of the gyrA and parC genes of nalidixic acid-resistant M. haemolytica were determined. All nalidixic acid-resistant strains possessed at least one amino acid substitution in each of the GyrA and ParC fragments investigated. These results suggest that M. haemolytica require at least one 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 their genotype by PFGE analysis was identical. This result indicates that fluoroquinolone-resistant M. haemolytica strains have clonally expanded.