Antimicrobial susceptibility and genomic analysis of Histophilus somni isolated from cases of bovine respiratory disease in Autralian feedlot cattle

• Published in: Veterinary microbiology 2022-07, Vol.270, p.109460-109460, Article 109460
• Main Authors: Alhamami, Tamara, Low, Wai Yee, Ren, Yan, Taylor, Kara, Khazandi, Manouchehr, Veltman, Tania, Venter, Henrietta, Carr, Mandi, Turni, Conny, Abraham, Sam, Trott, Darren J.
• Format: Article
• Language: English
• Subjects: Antimicrobial agents; Antimicrobial resistance; Antimicrobial susceptibility; Bovine respiratory disease; Breakpoints; Cattle; Chlortetracycline; Genetic diversity; Genomic analysis; Heart diseases; Histophilus somni; Meningoencephalitis; Minimum inhibitory concentration; Myocarditis; Nucleotide sequence; Oxytetracycline; Phylogenetics; Phylogeny; Respiratory diseases; Respiratory tract; Sequence analysis; Single-nucleotide polymorphism; Tetracyclines; Thromboembolism
• ISSN: 0378-1135; 1873-2542
• DOI: 10.1016/j.vetmic.2022.109460

Histophilus somni is a prevalent commensal organism of the upper respiratory tract of cattle and a major causative agent of bovine respiratory disease (BRD) and other syndromes including myocarditis and infectious thromboembolic meningoencephalitis. This study investigated the antimicrobial susceptibility and phylogenetic relationships of H. somni isolates obtained from lung, heart, and other tissues at post-mortem as well as nasal mucosa swabs from cases of BRD in Australian feedlots (2004–2019). Broth microdilution Minimal Inhibitory Concentration (MIC) assays were determined for 19 antimicrobials using three different media (CLSI approved Veterinary Fastidious Medium [VFM], Mueller-Hinton fastidious broth medium supplemented with yeast extract [MHF-Y] and Columbia Broth [CB] supplemented with 5% lysed horse blood). For all antimicrobials, MICs obtained using CB medium were identical or within 1 dilution step of the MICs obtained for VFM and MHF-Y media. Therefore, CB may be a suitable medium for H. somni antimicrobial susceptibility testing similar to MHF-Y medium. None of the 70 Australian H. somni isolates exhibited resistance to antimicrobials with CLSI breakpoints including those commonly used in the treatment of BRD in Australia (first-line tetracyclines [chlortetracycline and oxytetracycline], second-line macrolides [tulathromycin], and third-line extended-spectrum cephalosporin [ceftiofur]). Whole-genome sequence analysis of 65 H. somni isolates for genomic single nucleotide polymorphism differences identified four phylogenetic clusters, each containing isolates from different Australian states, feedlots and tissue sources that clustered together. These findings demonstrate limited genetic diversity and the absence of significant antimicrobial resistance among Australian isolates of H. somni isolated from feedlot cattle. •Histophilus somni a primary causative agent of bovine respiratory disease (BRD).•H. somni isolates samples highly collected from lung tissue compared with heart, other tissue and nasal swab.•Australian isolates of H. somni during (2004-2019) are pan-susceptible to the antimicrobial agents that are currently used to treat infections.•H. somni seems to be relatively homogeneous and showed no geographic clustering, in comparison with international isolates.