Previous studies had shown that short-term acute heat stress (HS) affected the host’s metabolism and intestinal microbiota independent of feed intake (FI) reduction, and long-term calorie restriction caused intestinal morphological injuries and gut microbial alterations. However, research on the effects of constant chronic HS on intestinal microbial composition and the roles of FI reduction played in is limited. This study aimed to investigate the effects of 7-day constant chronic HS on the composition of intestinal microbes in growing-finishing pigs, and its relationship with pigs’ performance, intestinal morphology, and ileal immune response. Twenty-four growing-finishing pigs (Duroc × Large White × Landrace, 30 ± 1 kg body weight) were randomly assigned to three treatments (n = 8), 1) thermal neutral (TN) conditions (25 ± 1 °C) with ad libitum FI, 2) HS conditions (35 ± 1 °C) with ad libitum FI, 3) pair-fed (PF) with HS under TN conditions to discriminate the confounding effects of dissimilar FI, and the FI was the previous day’s average FI of HS. The small intestinal segments (duodenum, jejunum, and ileum) and feces were collected on d 8. Results indicated that HS drastically declined (P < 0.05) average daily gain (ADG) and average daily feed intake (ADFI) (about 61%) in comparison with TN, and caused hyperpyrexia, meanwhile PF caused hypothermia. Morphological observation by light and electron microscopes showed that both HS and PF treatment decreased (P < 0.05) the villus and microvillus height compared with TN. Additionally, HS increased (P < 0.05) protein expression of heat shock protein 70 in the duodenum, jejunum, and ileum. Furthermore, the expression of tight junction protein zonula occluden-1 (ZO-1) in the duodenum and ileum, and Occludin in the ileum were enhanced (P < 0.05) compared with TN and PF. Moreover, HS significantly enhanced (P < 0.05) the mRNA relative expression of inflammatory cytokines (TLR-2, TLR-4, and tumor necrosis factor-α (TNF-α), IL-6, IL-8, PG1–5, β-defensin 2 (pBD-2)), mucins (mucin-1 and mucin-2) and P65 protein level in the ileal mucosa tissue. Intestinal microbiota analysis by 16S rRNA sequencing showed lower (P < 0.10) α diversity in both HS and PF, and a separated cluster of β diversity among groups. Compared with TN, HS but not PF mainly reduced (FDR < 0.05) Bacteroidetes (phylum), Bacteroidia (class) and elevated the proportions of Proteobacteria (phylum, FDR < 0.05), Bacillales (order, FDR < 0.05), Planococcaceae (family, FDR < 0.05), Kurthia (genus, FDR < 0.05), Streptococcaceae (family, FDR < 0.10) and Streptococcus (genus, FDR < 0.10). Notably, Lactobacillales (order) was decreased (FDR < 0.05) by PF alone. Furthermore, the Spearman correlation analysis indicated that the microbes prevalent in HS were positively (P < 0.05) associated with intestinal morphological injuries indicators and ileal immune response parameters, and the microbes reduced in HS were negatively (P < 0.05) with the performance data. Intestinal morphological injuries and ileal immune response caused by constant chronic HS independent of FI showed close connections with alterations in intestinal microbiota in growing-finishing pigs.

中文翻译：

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肠道微生物群组成的改变与持续慢性热应激下生长肥育猪的肠道形态受损和回肠免疫反应功能障碍相吻合

先前的研究表明，短期急性热应激 (HS) 会影响宿主的新陈代谢和肠道微生物群，而与采食量 (FI) 减少无关，而长期热量限制会导致肠道形态损伤和肠道微生物改变。然而，关于恒定慢性 HS 对肠道微生物组成的影响以及 FI 减少所起的作用的研究是有限的。本研究旨在探讨 7 天恒定慢性 HS 对生长肥育猪肠道微生物组成的影响，及其与猪的生产性能、肠道形态和回肠免疫反应的关系。24 头生长肥育猪（杜洛克 × 大白 × 长白猪，体重 30 ± 1 kg）被随机分配到三个处理组（n = 8），1) 热中性 (TN) 条件 (25 ± 1 °C)，随意 FI，2) HS 条件 (35 ± 1 °C)，随意 FI，3) 在 TN 条件下，成对喂食 (PF) 和 HS区分不同FI的混杂效应，FI是HS前一天的平均FI。在第 8 天收集小肠段（十二指肠、空肠和回肠）和粪便。结果表明，HS 急剧下降（P < 0.05）平均日增重（ADG）和平均日采食量（ADFI）（约 61%）与TN相比，引起高热，同时PF引起低体温。光镜和电镜形态学观察表明，HS和PF处理相比TN处理绒毛和微绒毛高度均降低（P < 0.05）。此外，HS 增加了（P < 0.05）十二指肠中热休克蛋白 70 的蛋白表达，空肠和回肠。此外，与TN和PF相比，十二指肠和回肠中紧密连接蛋白zonula occluden-1（ZO-1）和回肠中Occludin的表达增强（P <0.05）。此外，HS 显着增强（P < 0.05）炎性细胞因子（TLR-2、TLR-4 和肿瘤坏死因子-α（TNF-α）、IL-6、IL-8、PG1-5、回肠粘膜组织中的 β-防御素 2 (pBD-2))、粘蛋白（粘蛋白 1 和粘蛋白 2）和 P65 蛋白水平。通过 16S rRNA 测序进行的肠道微生物群分析显示，HS 和 PF 中的 α 多样性较低（P < 0.10），并且各组之间存在分离的 β 多样性簇。与TN相比，HS主要减少（FDR < 0.05）拟杆菌门（门）、拟杆菌门（纲），提高变形杆菌门（门，FDR < 0.05）、芽孢杆菌目（目，FDR < 0）的比例。05)、Planococcaceae (family, FDR < 0.05)、Kurthia (genus, FDR < 0.05)、Streptococcaceae (family, FDR < 0.10) 和Streptococcus (genus, FDR < 0.10)。值得注意的是，仅 PF 就减少了乳酸杆菌目（顺序）（FDR < 0.05）。此外，Spearman相关分析表明，HS中流行的微生物与肠道形态损伤指标和回肠免疫反应参数呈正相关（P <0.05），HS中减少的微生物与性能数据呈负相关（P <0.05）。独立于 FI 的持续慢性 HS 引起的肠道形态损伤和回肠免疫反应与生长肥育猪肠道微生物群的变化密切相关。10）和链球菌（属，FDR < 0.10）。值得注意的是，仅 PF 就减少了乳酸杆菌目（顺序）（FDR < 0.05）。此外，Spearman相关分析表明，HS中流行的微生物与肠道形态损伤指标和回肠免疫反应参数呈正相关（P <0.05），HS中减少的微生物与性能数据呈负相关（P <0.05）。独立于 FI 的持续慢性 HS 引起的肠道形态损伤和回肠免疫反应与生长肥育猪肠道微生物群的变化密切相关。10）和链球菌（属，FDR < 0.10）。值得注意的是，仅 PF 就减少了乳酸杆菌目（顺序）（FDR < 0.05）。此外，Spearman相关分析表明，HS中流行的微生物与肠道形态损伤指标和回肠免疫反应参数呈正相关（P <0.05），HS中减少的微生物与性能数据呈负相关（P <0.05）。独立于 FI 的持续慢性 HS 引起的肠道形态损伤和回肠免疫反应与生长肥育猪肠道微生物群的变化密切相关。Spearman相关分析表明，HS中流行的微生物与肠道形态损伤指标和回肠免疫反应参数呈正相关（P < 0.05），HS中减少的微生物与性能数据呈负相关（P < 0.05）。独立于 FI 的持续慢性 HS 引起的肠道形态损伤和回肠免疫反应与生长肥育猪肠道微生物群的变化密切相关。Spearman相关分析表明，HS中流行的微生物与肠道形态损伤指标和回肠免疫反应参数呈正相关（P < 0.05），HS中减少的微生物与性能数据呈负相关（P < 0.05）。独立于 FI 的持续慢性 HS 引起的肠道形态损伤和回肠免疫反应与生长肥育猪肠道微生物群的变化密切相关。