Australian Centre for Antimicrobial Resistance Ecology, School of Animal and Veterinary Sciences, The University of Adelaide, Roseworthy, SA, Australia.
Davies Livestock Research Centre, School of Animal and Veterinary Sciences, The University of Adelaide, Roseworthy, SA, Australia.
Vet Microbiol. 2022 Jul;270:109460. doi: 10.1016/j.vetmic.2022.109460. Epub 2022 May 13.
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.
唾液链球菌霍氏亚种是牛上呼吸道的一种常见共生体，也是牛呼吸道疾病（BRD）和其他综合征的主要病原体，包括心肌炎和感染性血栓性脑膜脑炎。本研究调查了从澳大利亚饲养场的 BRD 病例的肺部、心脏和其他组织以及鼻腔黏膜拭子中获得的 H. somni 分离株的抗菌敏感性和系统发育关系（2004-2019 年）。使用三种不同的培养基（CLSI 批准的兽医苛养菌培养基[VFM]、补充有酵母提取物的 Mueller-Hinton 苛养菌肉汤培养基[MHF-Y]和补充有 5%裂解马血的哥伦比亚肉汤[CB]）进行肉汤微量稀释最低抑菌浓度（MIC）测定，用于 19 种抗菌药物。对于所有抗菌药物，使用 CB 培养基获得的 MIC 与使用 VFM 和 MHF-Y 培养基获得的 MIC 相同或相差 1 个稀释度。因此，CB 可能是一种适合 H. somni 抗菌敏感性测试的培养基，类似于 MHF-Y 培养基。70 株澳大利亚 H. somni 分离株均未表现出对 CLSI 折点的抗菌药物耐药性，包括澳大利亚治疗 BRD 常用的药物（一线四环素[金霉素和土霉素]、二线大环内酯类[酒石酸泰乐菌素]和三线扩展谱头孢菌素[头孢噻呋]）。对 65 株 H. somni 分离株进行全基因组序列分析，以确定基因组单核苷酸多态性差异，确定了 4 个系统发育群，每个群都包含来自不同澳大利亚州、饲养场和组织来源的分离株，这些分离株聚集在一起。这些发现表明，从澳大利亚饲养场牛中分离出的 H. somni 分离株遗传多样性有限，且不存在明显的抗菌药物耐药性。
