Relationship between serotype and the antimicrobial susceptibility of Mannheimia haemolytica isolates collected between 1991 and 2010

The antimicrobial susceptibilities and serotype distribution of 310 Mannheimia haemolytica isolates obtained from cattle with bovine respiratory disease during 2002–2010 were investigated. Of the 310 isolates, 198 (63.9%) were resistant to at least one of the 16 tested antimicrobial agents. The resistance rates for ampicillin, amoxicillin, dihydrostreptomycin, kanamycin, oxytetracycline, doxycycline, chloramphenicol, thiamphenicol, nalidixic acid, enrofloxacin, and danofloxacin were 20.3%, 14.5%, 43.5%, 23.5%, 24.8%, 21.9%, 23.2%, 23.9%, 47.1%, 18.7%, and 18.7%, respectively. Almost 90% of the isolates belonged to three serotypes (serotypes A1, A2, and A6), and the relative prevalence of serotype A6 increased significantly over the last decade. Compared with bacteria belonging to other serotypes, bacteria belonging to serotype A6 exhibited a significantly higher antimicrobial resistance rates (χ2 test, p < 0.05).The results of this investigation provide useful information for understanding the serotype prevalence and antimicrobial resistance patterns of one of the major bacteriological agents implicated in pneumonic pasteurellosis.