Department of Biological Sciences, The Faculty of Science, University of Calgary, 2500 University Drive NW Calgary, Alberta, Canada.
Vet Microbiol. 2011 Jun 2;150(3-4):362-72. doi: 10.1016/j.vetmic.2011.02.051. Epub 2011 Mar 16.
Nasal swabs were collected at three time points from 2378 calves in four feedlots and cultured for Histophilus somni to assess genetic relatedness and tetracycline resistance. The proportions of animals carrying tetracycline resistant isolates were 0.32% at arrival, 14.82% at interim, and 0.80% at exit. The 606 H. somni isolates recovered were compared by pulsed-field gel electrophoresis (PFGE), screened for the presence of plasmids, and assessed for the tetracycline resistance genes tet(A), tet(B), tet(C), tet(E), tet(G), tet(H), tet(K), tet(L), tet(M) and tet(O) using multiplex polymerase chain reaction. Most of the isolates (98.6%) belonged to one of seven PFGE clusters (A-G) of closely related profiles with 77.7% of the isolates belonging to clusters C and D. Clusters A, B and E were associated with a higher proportion of tetracycline susceptible isolates. Genetic diversity of the isolates was highest at entry in the feedlot and lowest after the period when the animals received in-feed chlortetracycline (interim samples). Clusters A and E were more prominently represented at exit from the feedlot than other clusters. All resistant strains harboured the gene tet(H) while no other tetracycline resistance genes and no plasmids were detected with the methodology employed. It appears that genetic variability in H. somni in Alberta feedlots is low, dissemination likely occurs by clonal expansion, and resistance to tetracyclines is mediated by the tet(H) encoded efflux pump. Pulsotypes associated with tetracycline susceptible strains appear more common at exit suggesting that the in-feed oxytetracycline included throughout the feeding period is not sufficient to exert selective pressure for resistant strains.
从四个饲养场的 2378 头小牛中采集了三个时间点的鼻拭子，用于培养豪氏胸膜肺炎放线杆菌，以评估遗传相关性和四环素耐药性。到达时携带四环素耐药分离株的动物比例为 0.32%，中途为 14.82%，离开时为 0.80%。比较了回收的 606 株豪氏胸膜肺炎放线杆菌，通过脉冲场凝胶电泳（PFGE）进行比较，筛选质粒的存在，并使用多重聚合酶链反应评估四环素耐药基因 tet(A)、tet(B)、tet(C)、tet(E)、tet(G)、tet(H)、tet(K)、tet(L)、tet(M)和 tet(O)。大多数分离株（98.6%）属于 7 个密切相关的 PFGE 群（A-G）之一，其中 77.7%的分离株属于群 C 和群 D。群 A、B 和 E 与更高比例的四环素敏感分离株有关。在进入饲养场时，分离株的遗传多样性最高，在动物接受饲料中添加金霉素（中途样本）后最低。与其他群相比，群 A 和群 E 在离开饲养场时更为突出。所有耐药菌株均携带 tet(H)基因，而其他四环素耐药基因和质粒均未通过所采用的方法检测到。似乎阿尔伯塔省饲养场豪氏胸膜肺炎放线杆菌的遗传变异性较低，传播可能通过克隆扩张发生，四环素耐药性由 tet(H)编码的外排泵介导。与四环素敏感株相关的脉冲型似乎在离开时更为常见，这表明在整个饲养期添加的饲料中四环素不足以对耐药株施加选择压力。
