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Genetic characterization and antimicrobial susceptibility of Mannheimia haemolytica isolated from the nasopharynx of feedlot cattle

Klima, C.L., Alexander, T.W., Read, R.R., Gow, S.P., Booker, C.W., Hannon, S., Sheedy, C., McAllister, T.A., Selinger, L.B. (2011). Genetic characterization and antimicrobial susceptibility of Mannheimia haemolytica isolated from the nasopharynx of feedlot cattle, 149(3-4), 390-398. http://dx.doi.org/10.1016/j.vetmic.2010.11.018

Abstract

A surveillance study was undertaken to examine the population dynamics and antimicrobial resistance of Mannheimia haemolytica isolated from feedlot cattle. A total of 416 isolates were collected from the nasopharynx either upon entry or exit from two feedlots in southern Alberta, Canada. Isolates were serotyped, characterized by pulsed-field gel electrophoresis and tested for susceptibility to ten antimicrobial agents via disk diffusion. Resistant isolates were screened by PCR for select antimicrobial-resistance gene determinants. Isolates were highly diverse, with 335 unique pulsed-field profiles identified among 147 strongly related clusters (similarity ≥85%). Clonal spread of isolates throughout the feedlots was limited and no clear association was found between genetic relatedness of M. haemolytica and sampling event (entry or exit). Pulsed-field profiles sharing a common serotype and resistance phenotype tended to cluster together. The majority of isolates were identified as serotype 2 (74.5%) although both serotype 1 (11.9%) and 6 (12.7%) were detected. Only 9.54% of isolates exhibited antimicrobial resistance. Resistance to oxytetracycline was most prevalent (n=16), followed by ampicillin (n=10), and amoxicillin/clavulanic acid (n=7). Multi-drug resistance was observed in five isolates. The tetH gene was detected in all but two oxytetracycline resistant isolates. Other detectable resistance determinates included ermX and bla ROB-1. In the two feedlots examined, M. haemolytica exhibited considerable genetic diversity and limited resistance to common veterinary antibiotics. Garnering further information on the linkage between genotype and phenotype should contribute toward a better understanding of the pathogenesis and dissemination of M. haemolytica in feedlots. © 2010.

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