**5. Effect during stressful conditions**

The term "stress" is used to refer to the body's reactions to extrinsic or environmental factors that threaten homeostasis, or the body's normal state of physiological

### *Advances on Probiotics Utilization in Poultry Health and Nutrition DOI: http://dx.doi.org/10.5772/intechopen.113738*

equilibrium. Environmental stressor normally weakens the immune system of animals and increases their susceptibility to disease occurrence. Poultry birds respond to stress by modifying their behavior, biochemistry, and physiology in an effort to restore body's equilibrium. Based on the above, poultry animals normally suffer from one important stressful condition i.e., heat stress.

The deleterious effects of heat stress on the immunology, physiology, and microbiology of birds leads to negative impact on poultry industry. Genes like heat shock protein (HSP) are likely to have their expression altered in response to heat stress. Various studies have confirmed effective use of probiotics to supplementation in eliminating poultry heat stress. Ogbuagu et al. reported that combination of fisetin and probiotics, decreased erythrocyte osmotic fragility, lipid peroxidation and increased superoxide dismutase, thus reducing the effects of oxidative stress alterations in broiler chickens exposed to heat stress [34].

During chronic heat stress supplementing broiler with dietary probiotic of *B. subtilis* improved meat quality and alleviate oxidative deterioration of breast muscles [35]. A study similarly reported dietary *B. subtilis* (1 × 106 CFU/g feed) significantly improved broiler performance with respect to FCR and alleviation of immune response under both thermoneutral and heat stimulated conditions [36]. Terminal-RFLP analysis was used in another study to see how probiotic. The Cecal and jejunal microbiota of broiler chickens under heat stress is affected *Lactobacillus* strains. The study found out that there was no significant abundance in the microbial population but the supplementation did show higher prevalence of *Lactobacillus sp*. in heat stressed chickens in comparison with the controls [37].

Oxidative stress is a common consequence of heat stress in poultry. The combination of antioxidant compounds and probiotics offers a multifaceted approach to combating oxidative stress. One of the trace elements, selenium, is essential for the body's functioning. It shields red blood cells against the detrimental impacts of free radicals and constitutes a component of the robust antioxidant known as glutathione peroxidase. In summary, the normal development of enzymatic systems like superoxide dismutase and glutathione peroxidase is reliant on its presence. Khan et al. postulated that supplementing basal diet of broiler with selenium enriched probiotics (SP) (*S. cerevisiae* and *L. acidophilus*) enhanced antioxidant system to effectively inhibit hepatic oxidation during heat stress; probiotics alone were not as effective [38]. In another study SP, significantly improved body weight gain and bone health by up-regulating the expression of DIO2 and T3 in heat stressed broiler [39]. The effect of probiotic use in stressed poultry are summarized in **Table 1**.

Moreover, stress can disrupt the balance of the gut microbiota, affecting nutrient absorption and overall gut health. The role of probiotics in moderating this impact is explored in the following section.
