Host–microbiome interplay supports heat stress resilience in zebu calves

This study delineates the impact of heat stress (HS) on rumen microbial diversity and its association with host physiological responses in zebu calves, aiming to elucidate the microbiome-mediated mechanisms underlying thermal resilience. Six zebu calves (8–11 months; 120–150 kg) were sequentially subjected to thermoneutral (TN) [temperature-humidity index (THI) of 72.32] and HS (THI of 94.82) conditions for 21 days each, with a 10-day acclimatization and a 45-day washout interval. Physiological assessment under HS revealed significant (p < 0.05) elevations in rectal temperature, respiratory rate, cortisol, and IL-6 levels, along with significant (p < 0.05) declines in plasma aspartate aminotransferase (AST) activity and triglyceride levels. The volatile fatty acid (VFA) concentration remained unaltered, suggesting functional stability of the fermentative activity during HS. HS significantly altered the microbial diversity in the rumen of zebu calves, especially among bacteria, fungi, and protozoa, with archaeal changes limited to alpha diversity. HS led to a shift toward Pseudomonadota (notably Pseudomonas and Stenotrophomonas), whereas the TN condition favored Bacillota (e.g., Aerococcus and Soilbacillus). The genus-level fungal profile exhibited a high abundance of Thermothielavioides in the HS group and Schizosaccharomyces in the TN group. Species-level profiling indicated the enrichment of S. pombe in both groups. Similarly, the species-level protozoal profiles indicated a high abundance of Entodinium caudatum in both groups. Although the archaeal composition remained largely stable, the relative abundance of Methanobrevibacter (particularly M. ruminantium) was more prevalent in both groups. This study also indicated that HS enriched folate and amino acid biosynthesis pathways, while TN favored glycolytic metabolism. Microbial taxa like Stenotrophomonas and Aerococcus correlated strongly with stress and metabolic markers like IL-6, cortisol, HSP-90, AST, and triglycerides. Despite the increased abundance of opportunistic taxa under HS, the maintenance of VFA homeostasis and strong taxa–phenotype associations underscore the integral role of the rumen microbiome in mediating host adaptation to HS, thereby contributing to the thermal resilience of zebu calves.