The survival and cecal microbiota profile of broilers subjected to acute heat stress are contingent upon their body weight.

Heat stress (HS) is a growing challenge in poultry farming, driven by rising temperatures, which alters the gut and its microbiota. It is also assumed that body weight might be strongly correlated with a bird's ability to tolerate HS. However, there is no clear evidence of the potential interaction between HS exposure, body weight, and their effects on the host's microbiota in the short term. Hence, this study evaluated the effects of acute HS on the cecal bacterial community in chickens, relative to host body weight, and explored potential links between the microbiota and HS resistance. A total of ninety (n = 6 replicates of 5 birds) 36-day-old birds were grouped by body weight class (BWC): Low (2373.9 ± 32.1 g), Med (2681.1 ± 11.2 g), and High (3009.5 ± 36.0 g), and were subsequently either exposed to acute HS (35 ± 1 °C, 8 hours) or kept under thermoneutral conditions (CON). At the end of the trial, their cecal microbiota were subjected to metataxonomic analysis. Results revealed that, compared with their High-BWC counterparts (median survival time: 300 min), birds in the Low-BWC group exhibited greater resistance to HS (median survival time: 359 min). Metataxonomic analyses showed that acute HS increased evenness indices in the HS_Low group compared with the CON_Low group. The HS_Low group also demonstrated significantly higher evenness than the HS_High group. Conversely, beta-diversity analyses revealed no significant effects of HS or BWC. Bacterial communities were dominated by Bacillota and Bacteroidota phyla, with Bacteroides fragilis, Blautia hominis, and Enterococcus cecorum as the most abundant species. More microbial taxa were enriched in the HS group than in the CON groups. Identified biomarkers included twelve for the HS_Low, five for the HS_Med, and three for the HS_High, with Mediterraneibacter glycyrrhizinilyticus, Pseudoflavonifractor capillosus, and Clostridium phoceensis being the most significant for each group, respectively. Relative abundances of Erysipelotrichia, Erysipelotrichales, and Erysipelotrichaceae exhibited a linear decrease with the ratio between initial and final BW. Microbial co-occurrence networks were resilient to acute HS, with no structural differences between the CON and HS groups. Overall, these findings suggest that the BWC may serve as a reliable predictor of acute HS survival in broiler chickens. However, the greater HS resistance observed in lighter birds may be associated with rapid changes in cecal microbiota evenness during exposure. Besides, Erysipelotrichaceae may serve as a microbial indicator of HS resistance in broilers, warranting further investigation.