Lysozyme and Bacillus subtilis improve growth performance, nutrient utilization, physiological stress indices, intestinal integrity, and gene expression of heat-stressed broilers

Feed additives play a crucial role in enhancing the poultry's overall performance, whether by meeting their needs or helping to overcome various challenges, especially during heat stress. This study examined the effects of adding lysozyme and Bacillus subtilis on growth indices, lipid profiles, immune responses, gut integrity, and gene expression in heat-stressed broilers. A total of 600 one-day-old (Ross 308) male chicks were randomly allocated into four treatment groups (six replicates) as follows: CON, a basal diet without feed additives; B.S, a basal diet supplemented with B. subtilis (500 mg/kg diet); LYS, a basal diet supplemented with lysozyme (150 mg/kg diet); and BSLY, a basal diet supplemented with both B. subtilis and lysozyme (500 mg/kg and 150 mg/kg diet, respectively). Chickens were exposed to a temperature of 33 °C for 4 h daily during the experimental period. Results indicated that broilers receiving a diet supplemented with BSLY showed higher body weight gain and lower feed conversion ratio than the control (p < 0.05). The carcass characteristics and nutrient digestibility of the BSLY group were also improved. Total cholesterol decreased in chickens fed BSLY and B.S, while low-density lipoprotein (LDL) levels decreased in the LYS group. compared to the control group; however, high-density lipoprotein (HDL) increased in the BSLY, B.S, and LYS groups. Additionally, the addition of BSLY and B.S enhanced the immune response by increasing the levels of immunoglobulin G (IgG), immunoglobulin M (IgM), and the relative weight of the Bursa of Fabricius. Furthermore, gut integrity improved via expanding the ileum villus height in chickens fed BSLY, B.S, and LYS , as well as the counts of Bacillus spp. and Lactobacillus in the broilers fed BSLY and B.S. Dietary BSLY, B.S, and LYS modulated gene expression, via up-regulation of insulin-like growth factor 1 (IGF1), superoxide dismutase (SOD), glutathione peroxidase (GPx), and interferon-gamma (IFN-γ) gene expression compared to the control group, while the highest SOD and IGF1 genetic modification was in the group that received BSLY. In conclusion, adding B. subtilis-lysozyme mixture can have a positive impact on growth performance indices, immune response, gut health, and gene expression, thereby increasing heat stress resistance of broiler chickens.