Dietary heat-killed Limosilactobacillus ingluviei C37 postbiotic improves growth performance and modulates stress-related gene expression in heat-stressed broiler chickens

• HK-LIC37 postbiotic promotes growth and gut health in broilers under HS • HK-LIC37 modulates hepatic antioxidants and stress-related genes • HK-LIC37 serves as a promising additive to mitigate HS in broiler chickens This study aimed to evaluate the efficacy of dietary supplementation with heat-killed Limosilactobacillus ingluviei C37 (HK-LIC37) postbiotic on growth performance and expression of antioxidant enzymes, intestinal inflammation and barrier function, and cellular stress marker genes in heat-stressed broilers. A total of 360 male broilers Ross 308 were randomly allocated to 6 groups with 6 replicates per group in a completely randomized design (CRD). The first group was maintained under thermoneutral zone (TNZ) conditions (21 ± 1°C), received the basal diet, and served as the control. Groups 2 – 6 were exposed to cyclic chronic heat stress (HS) (32 ± 1°C for 5 h daily) from day 15 to day 42 and received one of the following diets: negative control (NC) with basal diet only, positive control (PC) with basal diet + 50 mg/kg zinc bacitracin, or basal diet supplemented with HK-LIC37 postbiotic at levels of 1 × 10 7 , 1 × 10 8 , and 1 × 10 9 cfu/kg diet for the treatment groups. The results revealed that HS negatively affected growth performance and upregulated the expression of genes related to inflammation and cellular stress markers ( TNF-α, NF-κB , and HSP70 ). Interestingly, dietary supplementation with HK-LIC37 postbiotic improved growth performance and modulated gene expression in heat-stressed broilers. Specifically, the postbiotic enhanced antioxidant enzyme defense by upregulating GPx expression, while alleviating cellular stress and inflammation markers through downregulation of HSP70 and NF-κB in the liver, along with their downstream pro-inflammatory cytokines TNF-α and IL-6 in jejunal tissue. Additionally, HK-LIC37 strengthened intestinal mucosal and barrier function by upregulating the MUC-2 and OCLN genes in jejunal tissue. The results suggest that HK-LIC37 postbiotic has the potential to alleviate the negative effects of HS and could serve as a natural feed additive to restore gut health and performance under challenging climatic conditions.