Heat stress and feeding effects on the mucosa-associated and digesta microbiome and their relationship to plasma and digesta fluid metabolites in the jejunum of dairy cows

Abstract

The intestinal microbiota plays a pivotal role in digestive processes and maintains gut health and intestinal homeostasis. These functions may be compromised by increased environmental heat which in turn reduces feed intake and gut integrity, while activating the intestinal immune system. However, it is not known whether high ambient temperatures, causing heat stress (HS) to dairy cows, disturb the eubiosis of the microbial community and if so, to which extent the reduction in feed intake and the impairment of circulating and intestinal metabolites induced by HS account for the alterations of the jejunal microbiota. To address these questions, jejunal digesta, mucosa, and plasma samples from cows exposed to heat stress (HS: 28°C, temperature-humidity-index (THI) = 76, n = 10), or control conditions (CON: 16°C, THI = 60, n = 10), or pair-feeding (PF: 16°C, THI = 60, n = 10) for 7 days were collected. Digesta fluids were examined for pH, acetate, non-esterified fatty acids (NEFA), glucose, and lactate, whilst plasma metabolites were analyzed for glucose, lactate, beta-hydroxybutyrate (BHB), triglycerides, NEFA, creatinine and urea. The microbiota composition of digesta and mucosa samples were analyzed by 16S rRNA sequencing. The alpha diversity was higher in mucosa than digesta, but not affected by high ambient temperatures. However, the mucosa-associated microbiota appears more prone to ambient heat than the digesta microbiome. Furthermore, HS cows showed a greater mucosal abundance of Bifidobacteriaceae, Succinivibrionaceae UCG-001, Clostridia and a trend for Lactobacillus compared to CON cows. In the digesta, lower abundance of Colidextribacter and a trend for lower Lachnospiraceae UCG-008 were found in HS than CON cows. Several correlations between plasma or intestinal metabolites and microbiota were elucidated, including Methanobacteriaceae correlating positively with plasma BHB and digesta glucose concentrations. The shifts in the microbial profile of the small intestine give some implications for the adaptive responses under HS conditions. Moreover, the reduction in feed intake during HS had non-negligible effects on microbial diversity and the abundance of certain taxa, underpinning the importance of nutrient supply on maintaining intestinal homeostasis.