Abstract
This feeding trial aimed to assess the impacts of B. subtilis strains PB6 and QST713 on growth, some blood parameters, immune response and gut integrity in broilers. 160 day-old broiler chicks (Avian 48) randomly allotted into four treatments, each subdivided into four replicates (10 birds per replicate). Birds of group 2 and 3 were accessed to the basal diet treated with 100 g/ ton Bacillus subtilis QST713 (1 ×1010 CFU/g) and 500 g Bacillus subtilis PB6 (2×108 CFU/g) / ton respectively, while birds of group 4 were fed on the basal diet treated with the half dose of both previous strains. No significant differences were observed in final body weight (BW), body weight gain (BWG), feed conversion ratio (FCR), or feed intake (FI) between the supplemented groups and the control. There were no significant differences in the biochemical indices between the supplemented groups and the control. Bacillus subtilis supplementation significantly upregulated (P < 0.0001) the expression of antioxidant-related genes CAT (catalase) and SOD1 (superoxide dismutase), indicating improved antioxidant efficiency in the supplemented groups. Expression of pro-inflammatory cytokines IL-1β and IL-6 was significantly downregulated (P < 0.0001) in the groups supplemented with Bacillus subtilis, suggesting an anti-inflammatory effect. Bacillus subtilis PB6 had the longest jejunal villi (P < 0.0001). The smallest width was found in the combination group, whilst the maximum width was found in the birds who received Bacillus subtilis PB6 compared to the control and Bacillus subtilis QST713 receiving groups. However, the deepest crypts were found in the combination group. Bursa of Fibracious showed the largest follicular area in the groups received mixture of both probiotics. In conclusion, dietary supplementation with Bacillus subtilis strains (PB6 and QST713) can improve immune responses, gut health, and antioxidant status in broilers without negatively impacting growth performance or health. The combination of both strains particularly promoted the development of the immune organs and systemic immune responses, suggesting these probiotics enhance both local gut immunity and systemic immune function.

Key words: Broilers, Bacillus subtilis, Performance, Blood parameters, Immunity, Gut integrity