Characterization of antimicrobial resistance in Pasteurella multocida isolated from Albertan feedlot cattle

Bovine respiratory disease (BRD) is an important health issue for the North American feedlot industry, causing substantial financial losses due to morbidity and mortality. While polymicrobial in nature, Pasteurella multocida is considered a principal bacterial pathogen implicated in BRD. Antimicrobials are widely used to prevent (metaphylaxis) BRD in newly received feedlot cattle, however concerns regarding overuse and the development of resistance in pathogens exists. The objective of this study was to evaluate antimicrobial resistance in P. multocida isolated from Albertan feedlot cattle that were sampled 8 years apart. A total of 134 isolates were collected from individual animals between the years 2008 to 2009 (Cohort 1) and 133 isolates between 2015-2016 (Cohort 2) from Albertan feedlots that employed similar management strategies (i.e. metaphylaxis and vaccination of cattle on arrival). Antimicrobial resistance patterns changed over the 8-year time period and was lower (P<0.05) in cohort 1 compared to cohort 2 for oxytetracycline (3% vs 80%), neomycin (4% vs 63%) and tulathromycin (4% vs 78%). No isolates were resistant against enrofloxacin. Coinciding with elevated resistance in Cohort-2 isolates was reduced genetic similarity, and observation of the spread of resistant clones between cattle. When evaluated for resistance genes by PCR, the majority (98%) of oxytetracycline-resistant isolates carried tet(H) while only 16 out of the 106 tulathromycin-resistant isolates from cohort 2 carried a known macrolide resistance gene. Therefore, the genomes of 9 tulathromycin-resistant isolates were sequenced, leading to the identification a conserved gene cluster that was present in all isolates with tulathromycin resistance but unknown macrolide resistance genes. One of the genes was a novel methylase, which when cloned into Escherichia coli, doubled the minimum inhibitory concentration against tulathromycin. This study showed that macrolide resistance in P. multocida increased over an 8 year span, and implied that antimicrobial use selected for resistant bacteria, thereby reducing diversity of P. multocida in feedlots. Given a lack of concordance in resistance genotype and phenotype, surveillance studies will benefit from a combination of culture dependent and independent methods to effectively evaluate resistance in feedlots.