title: Antimicrobial susceptibility of Histophilus somni isolated from clinically affected cattle in Australia.
abstract: This study investigated antimicrobial resistance traits, clonal relationships and epidemiology of Histophilus somni isolated from clinically affected cattle in Queensland and New South Wales, Australia. Isolates (n = 53) were subjected to antimicrobial susceptibility testing against six antimicrobial agents (ceftiofur, enrofloxacin, florfenicol, tetracycline, tilmicosin and tulathromycin) using disc diffusion and minimum inhibitory concentration (MIC) assays. Clonal relationships were assessed using repetitive sequence PCR and descriptive epidemiological analysis was performed. The H. somni isolates appeared to be geographically clonal, with 27/53 (47%) isolates grouping in one cluster from one Australian state. On the basis of disc diffusion, 34/53 (64%) isolates were susceptible to all antimicrobial agents tested; there was intermediate susceptibility to tulathromycin in 12 isolates, tilmicosin in seven isolates and resistance to tilmicosin in one isolate. Using MIC, all but one isolate was susceptible to all antimicrobial agents tested; the non-susceptible isolate was resistant to tetracycline, but this MIC result could not be compared to disc diffusion, since there are no interpretative guidelines for disc diffusion for H. somni against tetracycline. In this study, there was little evidence of antimicrobial resistance in H. somni isolates from Australian cattle. Disc diffusion susceptibility testing results were comparable to MIC results for most antimicrobial agents tested; however, results for isolates with intermediate susceptibility or resistance to tilmicosin and tulathromycin on disc diffusion should be interpreted with caution in the absence of MIC results.
authors.affiliation: Biosecurity Sciences Laboratory, Biosecurity Queensland, Brisbane, Queensland 4108, Australia; The University of Queensland, School of Veterinary Science, Gatton, Queensland 4343, Australia.
authors.forename: Lauren K
authors.initials: LK
authors.lastname: Goldspink
authors.affiliation: Biosecurity Sciences Laboratory, Biosecurity Queensland, Brisbane, Queensland 4108, Australia.
authors.forename: Joanne L
authors.initials: JL
authors.lastname: Mollinger
authors.affiliation: The University of Queensland, School of Veterinary Science, Gatton, Queensland 4343, Australia; The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Centre for Animal Science, St Lucia, Queensland 4072, Australia.
authors.forename: Tamsin S
authors.initials: TS
authors.lastname: Barnes
authors.affiliation: The University of Queensland, School of Veterinary Science, Gatton, Queensland 4343, Australia.
authors.forename: Mitchell
authors.initials: M
authors.lastname: Groves
authors.affiliation: The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Centre for Animal Science, St Lucia, Queensland 4072, Australia.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Mahony
authors.affiliation: The University of Queensland, School of Veterinary Science, Gatton, Queensland 4343, Australia. Electronic address: gibson.j@uq.edu.au.
authors.forename: Justine S
authors.initials: JS
authors.lastname: Gibson
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001315:Australia / Q000453:epidemiology; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology / Q000382:microbiology*; D052978:Disk Diffusion Antimicrobial Tests / Q000662:veterinary; D024881:Drug Resistance, Bacterial; D005260:Female; D008297:Male; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D015661:Pasteurellaceae / Q000187:drug effects*; D016871:Pasteurellaceae Infections / Q000453:epidemiology / Q000382:microbiology / Q000662:veterinary*
chemical_list: D000900:Anti-Bacterial Agents
keywords: Antimicrobial susceptibility; Bovine respiratory disease; Disc diffusion; Histophilus somni; Minimum inhibitory concentration