Honors Program


Date of this Version

Spring 3-11-2019

Document Type



Deal, C. 2019. Characterization of Multidrug Resistant Histophilus somni Associated with Bovine Respiratory Disease. Undergraduate Honors Thesis. University of Nebraska-Lincoln.


Copyright Caitlyn Deal 2019.


Bovine Respiratory Disease Complex (BRDC) is a multifactorial disease complex of significance affecting North American feedlot cattle, causing great economic loss. Histophilus somni, a primary pathogen causing BRDC, has shown recent evidence of decreased susceptibility to common antimicrobials approved for BRDC therapy. To combat the increase in resistance and economic loss and facilitate judicious treatment, rapid identification of genes conferring resistance and transmissible elements enabling dissemination is necessary. This study aimed to extend the use of a real time hydrolysis probe polymerase chain reaction (PCR) assay developed to identify macrolide genes conferring resistance in Mannheimia haemolytica to effectively identify these resistance determinants present in Histophilus somni. Susceptible (n=23) and resistant (n=22) H. somni field isolates were purified, extracted and run on PCR assay to analyze the efficacy of identifying macrolide resistance genes erm(42), mrs(E), mph(E) and the integrative conjugative element tetracycline repressor, ICEtetR, conferring resistance to tetracyclines. Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) and Fourier-transform Infrared Spectroscopy (IR) were utilized to create phylogenetic trees visualizing similarities and groupings between susceptible and resistant populations. Real time PCR was successful in identifying the presence of resistance determinants in H. somni, with presence of determinants in agreement with 80% (36/45) of MIC-determined susceptibilities to tulathromycin and 82% (37/45) to tetracycline antibiotics. Agreement between phenotype and genotypes were good with a kappa statistic of K=0.69 for both resistance determinants. This PCR assay can be a powerful diagnostic tool used in conjunction with classic MIC testing to determine both phenotypic and genotypic expression of antibiotic susceptibilities in individual bacterial strains and potentially applied to clinical samples for surveillance and rapid testing.