Heat stress (HS) affects the ruminal microbiota and decreases the lactation performance of dairy cows. Because HS decreases feed intake, the results of previous studies were confounded by the effect of HS on feed intake. This study examined the direct effect of HS on the ruminal microbiota using lactating Holstein cows that were pair-fed and housed in environmental chambers in a 2 × 2 crossover design. The cows were pair-fed the same amount of identical total mixed ration to eliminate the effect of feed or feed intake. The composition and structure of the microbiota of prokaryotes, fungi, and protozoa were analyzed using metataxonomics and compared between two thermal conditions: pair-fed thermoneutrality (PFTN, thermal humidity index: 65.5) and HS (87.2 for daytime and 81.8 for nighttime). The HS conditions altered the structure of the prokaryotic microbiota and the protozoal microbiota, but not the fungal microbiota. Heat stress significantly increased the relative abundance of Bacteroidetes (primarily Gram-negative bacteria) while decreasing that of Firmicutes (primarily Gram-positive bacteria) and the Firmicutes-to-Bacteroidetes ratio. Some genera were exclusively found in the heat-stressed cows and thermal control cows. Some co-occurrence and mutual exclusion between some genera were also found exclusively for each thermal condition. Heat stress did not significantly affect the overall functional features predicted using the 16S rRNA gene sequences and ITS1 sequences, but some enzyme-coding genes altered their relative abundance in response to HS. Overall, HS affected the prokaryotes, fungi, and protozoa of the ruminal microbiota in lactating Holstein cows to a different extent, but the effect on the structure of ruminal microbiota and functional profiles was limited when not confounded by the effect on feed intake. However, some genera and co-occurrence were exclusively found in the rumen of heat-stressed cows. These effects should be attributed to the direct effect of heat stress on the host metabolism, physiology, and behavior. Some of the “heat-stress resistant” microbes may be useful as potential probiotics for cows under heat stress.

中文翻译：

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热应激影响多域瘤胃微生物群和一些功能特征，与其对泌乳奶牛采食量的影响无关

热应激（HS）影响瘤胃微生物群并降低奶牛的泌乳性能。由于 HS 会降低采食量，因此之前的研究结果被 HS 对采食量的影响所混淆。本研究使用以 2 × 2 交叉设计成对饲养并饲养在环境室中的泌乳荷斯坦奶牛，检查了 HS 对瘤胃微生物群的直接影响。奶牛被配对喂养相同数量的相同总混合日粮，以消除饲料或采食量的影响。使用元分类学分析了原核生物、真菌和原生动物的微生物群的组成和结构，并在两种热条件之间进行了比较：成对喂养的热中性（PFTN，热湿度指数：65.5）和 HS（白天为 87.2，夜间为 81.8）。HS 条件改变了原核微生物群和原生动物微生物群的结构，但没有改变真菌微生物群的结构。热应激显着增加了拟杆菌门（主要是革兰氏阴性菌）的相对丰度，同时降低了厚壁菌门（主要是革兰氏阳性菌）的相对丰度和厚壁菌门与拟杆菌门的比例。一些属仅在热应激奶牛和热控制奶牛中发现。某些属之间的一些共生和互斥也仅针对每种热条件而被发现。热应激并未显着影响使用 16S rRNA 基因序列和 ITS1 序列预测的整体功能特征，但一些酶编码基因响应 HS 改变了它们的相对丰度。总体而言，HS 影响了原核生物、真菌、泌乳荷斯坦奶牛瘤胃微生物群和原生动物的差异程度不同，但在不受采食量影响的情况下，对瘤胃微生物群结构和功能谱的影响是有限的。然而，一些属和共生仅在热应激奶牛的瘤胃中发现。这些影响应归因于热应激对宿主代谢、生理和行为的直接影响。一些“抗热应激”微生物可用作热应激奶牛的潜在益生菌。这些影响应归因于热应激对宿主代谢、生理和行为的直接影响。一些“抗热应激”微生物可用作热应激奶牛的潜在益生菌。这些影响应归因于热应激对宿主代谢、生理和行为的直接影响。一些“抗热应激”微生物可用作热应激奶牛的潜在益生菌。