According to the United Nations Intergovernmental Panel on Climate Change (IPCC) report, the burning of fossil fuels and deforestation (Gray, 2007; Jianbin, 2013) accelerate the pace of global warming, which will be a big challenge for the dairy industry in China (Zhang et al., 2020), impacting the success of its dairy industry. Shandong Province in East China has a temperate monsoon climate with high summer temperatures, and cows in this region are severely affected by heat stress. This has led to a decrease in milk production and feed intake and an increase in the incidence of intramammary bacterial infection (IMI) (West, 2003; Rakib et al., 2020). Heat stress also lowers immunity and antioxidant capacity (Tao et al., 2012; Du et al., 2022) and changes the bacterial composition and metabolites of the rumen (Zhao et al., 2019). This is detrimental to the health of cows and causes serious losses to the dairy industry.

The gastrointestinal tract of cows is a complex ecosystem that has a strong impact on health (Zeineldin et al., 2018a). The rumen, the first stomach of ruminants, has a complex and diverse microbiota that can digest cellulose and provide nutrients to the organism (Bickhart and Weimer, 2018). The rumen microbiota can also stimulate and improve immune responses (Zhang et al., 2021). Recent studies have shown that the negative effects of heat stress on cows can be ameliorated by adding probiotics or other additives to the feed (Liu et al., 2014; Ye et al., 2016; Shah et al., 2020; Li et al., 2021a). These studies have shown varying degrees of improvement in milk production, immunity, and antioxidant capacity, as well as changes in the composition of the rumen microbiota and metabolites following the addition of these products. This may be due to the critical role played by host-rumen-microbiota interactions, and these additives alter the composition of the rumen flora and its microbial metabolites, thereby improving the health of the cow. The same argument is made in a review by Zeineldin et al. (2018b).

Our previous studies showed that Saccharomyces cerevisiae cultures could improve the performance, antioxidant capacity, and immunity of heat-stressed cows and reduce corporate losses (Du et al., 2022). Previous studies on heat-stressed cows have centered on rumen microbial diversity and metabolomics, and this experiment instead focused on rumen microbial diversity, metagenomics, and metabolomics. The objective of this study was to determine the effect of the addition of S. cerevisiae cultures on rumen microbial composition, rumen microorganisms’ function, and rumen metabolites.

To understand the mechanism by which S. cerevisiae cultures alleviate heat stress in dairy cows, the experiment investigated the effects of S. cerevisiae cultures on the microbial composition, metagenome, and metabolome of the rumen.

Previous studies indicate that the species diversity and abundance of rumen microorganisms play a critical role in maintaining host physiology (Xu et al., 2021). While Chao1 and ACE indices are primarily used to measure species richness, the Simpson and Shannon indices combine the richness and evenness of a species. Our study found that rumen bacterial abundance and diversity were higher in cows that were fed with the first yeast culture than in the other two groups, although these findings were not statistically significant. This may be due to the increased diversity and abundance of rumen bacteria after the addition of S. cerevisiae culture. The environment within the rumen can change to some extent and may affect the bacterial species involved (Ghazanfar et al., 2017). Our results showed that Bacteroidetes, Proteobacteria, and Firmicutes were the dominant phyla in the rumen of all three groups of dairy cows, which supports the findings of prior studies (Xue et al., 2019). The abundance of Firmicutes was highest in the D-A group, while the abundance of Proteobacteria was lowest in the D-A group; however, these differences were not significant. Firmicutes are associated with the degradation of structural polysaccharides (Zened et al., 2013), which may impact the breakdown of cellulose in the rumen. Zou et al. (2019) found that yaks with lower feed conversion efficiency also have a lower relative abundance of Firmicutes in the rumen. In contrast, Proteobacteria is associated with increased disease risk and an elevated abundance of these bacteria correlates with gut inflammation. Ruminococcus and Firmicutes are involved in degrading fiber from cellulose-rich feeds (Evans et al., 2011), while Butyrivibrio is one of the few genera that can utilize xylan and pectin and degrade hemicellulose, hydrogenate fatty acids, and protein (Bryant and Small, 1956; Orpin et al., 1985; Paillard et al., 2007). Moryella, a member of the Lachnospira family of Firmicutes, can produce short-chain fatty acids through fermentation of dietary polysaccharides (Laffin et al., 2019). Short-chain fatty acids are important substrates for regulating gastrointestinal tract epithelial cells and are able to modulate the inflammatory response (Guo et al., 2020). Our study found a significant increase in the abundance of several genera involved in cellulose catabolism in the D-A group, some of which were also able to produce volatile fatty acids, which decreased the abundance of Proteobacteria. These findings indicated that the addition of S. cerevisiae culture increased cellulose degradation and reduced inflammation in heat-stressed cows.

The trial assessed the mechanism by which S. cerevisiae cultures affect the rumen of heat-stressed cows by metagenome and found ten pathways that differed significantly between the three groups. Fatty acid biosynthesis, fatty acid metabolism, peroxisome proliferator-activated receptor (PPAR) signaling, and ferroptosis and biotin metabolism were all enriched in the treatment groups. Yeast is known as a natural source of B vitamins. According to let’s eat right to keep fit by Adelle Davis, yeast is one of the four foods that contain complete vitamin B. S. cerevisiae culture is a product obtained from S. cerevisiae by fermentation and concentration, it is also rich in high levels of vitamin B, and studies indicate that vitamin B compounds can increase milk production in dairy cows (Chen et al., 2011). Biotin, a water-soluble B vitamin, is an essential nutrient in the rumen of dairy cows and a required cofactor for five carboxylases. These biotin-dependent carboxylases participate in several metabolic pathways, including fatty acid synthesis and fatty acid oxidation, and biotin or carboxylase deficiency can lead to metabolic dysfunction. Saika et al. (2019) and Lauridsen (2020) found that fatty acids are involved in intestinal mucosal barrier function and oxidative stress and inflammatory responses (Saika et al., 2019; Lauridsen, 2020). By activating fatty acid receptors, they can promote anti-inflammatory and neuronal responses and regulate immune homeostasis (Kimura et al., 2019). The PPAR is also activated by fatty acids and involved in regulating lipid metabolism and inflammation. In addition, PPAR signaling can inactivate NF-κB, promote antioxidant enzyme expression, and reduce the concentration of reactive oxygen species, thus improving antioxidant capacity (Korbecki et al., 2019). Cellulolytic bacteria in the rumen require biotin for growth, and Abel et al. (2001) showed that biotin improves fiber fermentation in the rumen. Thus, we speculated that the addition of S. cerevisiae culture increased B vitamin levels in the rumen, which increased biotin metabolism and production, elevated fatty acid synthesis, activated PPAR signaling, and improved the immune response and antioxidant capacity of the cows. The increase in biotin also elevated the abundance of cellulolytic Firmicutes in the rumen and improved the digestibility of the feed.

Ferroptosis is an iron-dependent type of programed cell death caused by massive lipid peroxidation-mediated membrane damage. This process is distinct from apoptosis, cellular necrosis, and cellular autophagy (Stockwell et al., 2017; Tang et al., 2019). In most cases, ferroptosis causes an inflammatory response in the body that induces cell death and is harmful to the organism. However, there is evidence that ferroptosis is a double-edged sword and may protect cells from inflammation under certain conditions (Sun et al., 2020). Two groups of cows fed with S. cerevisiae cultures in our study were enriched in the ferroptosis pathway, with significant differences between the D-A and control groups, which may be related to increased fatty acid metabolism.

The taste transduction pathway in mammals is not only capable of producing taste but is also expressed in gastrointestinal tract tissues so may play an important role in resisting microbial infections, regulating nutrient absorption, and maintaining internal homeostasis (Yangyu et al., 2017). The receptors for sweetness, freshness, and bitterness are all G protein-coupled, and Minic et al. (2005) found a high degree of homology between the yeast pheromone signaling pathway and mammalian G protein-coupled receptors. G protein-coupled receptors are also expressed in S. cerevisiae, which may explain why metabolites are enriched in the taste transduction pathway. The primary breakdown products of cellulose are sucrose and maltose, and according to the sequencing results, the increased abundance of cellulolytic bacteria is associated with elevated maltose levels. G protein-coupled receptors respond to maltose stimulation (Zhao et al., 2003) and release acetylcholine. These results suggest that S. cerevisiae cultures may enrich the ruminal taste transduction pathways involved in maintaining internal homeostasis.

The pentose phosphate pathway is the primary pathway for oxidative glucose catabolism in animals and microorganisms and regulates cellular energy supply and cellular respiration, improving metabolic efficiency (Li et al., 2021b). Ruminococcus metabolizes pentose via the pentose phosphate pathway, so the increased abundance of Ruminococcus in this study may account for the upregulation of metabolites associated with the pentose phosphate pathway in the D-A group. The pentose phosphate pathway produces NADPH, which is essential for antioxidant capacity (Patra and Hay, 2014), acting as a hydrogen donor for fatty acid synthesis, participating in fatty acid synthesis, and providing energy to the body (Ohlrogge and Jaworski, 1997). Taurine is a non-essential amino acid that cannot function as a direct energy source (Chesney, 1985) but can provide protection against oxidative stress by increasing superoxide dismutase and glutathione peroxidase activity (Nonaka et al., 2001). Significant enrichment of the pentose phosphate pathway and increased taurine content after the addition of S. cerevisiae cultures improved the antioxidant capacity of heat-stressed cows. Glycochenodeoxycholic acid is a hydrophobic bile salt that accumulates in the liver during cholestasis and can induce apoptosis in hepatocytes at certain concentrations. In the D-A group, glycochenodeoxycholic acid production was downregulated, which may have relieved stress on the liver and ensured the health of the organism.

This trial studied the effects of S. cerevisiae culture-supplemented diets of heat-stressed cows on rumen microbial composition, metagenomics, and metabolomics and identified a strong relationship between supplementation and vitamin B. As shown in Figure 8, yeast cultures are rich in vitamin B. Supplementation with S. cerevisiae culture enriched the rumen in biotin, a water-soluble vitamin B. Biotin can act as a carboxylase cofactor, participating in the synthesis and metabolism of fatty acids, which in turn can activate PPAR and improve the antioxidant capacity and immunity of the organism. In addition, most cellulolytic bacteria require vitamin B for growth, which may account for the increased abundance of cellulolytic bacteria such as Ruminococcus and Butyrivibrio. These bacteria stimulate cellulose catabolism, upregulate metabolites in the pentose phosphate pathway, increase primary bile acid biosynthesis and taste transduction pathway signaling, and positively impact the health of cows. Our findings indicate that S. cerevisiae cultures enriched with vitamin B provide protection against heat stress and should be considered as an anti-heat stress feed additive in production.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found at: https://www.ncbi.nlm.nih.gov/, PRJNA834564.

The animal study was reviewed and approved by Ethical Commission of Shandong Agricultural University.

DD, PC, CW, and ZH conceived the project and designed the protocol. DD and WJ performed the experiments. DD, LF, and YZ wrote the manuscript. All authors read and approved the final manuscript.

PC was employed by Beijing Enhalor International Tech Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.