AimsTo determine the efficacy of Bacillus subtilis strain in prevention of heat stress-related complications in rats. Methods and ResultsMale Sprague-Dawley rats weighing 250-300g were treated by oral gavage with B.subtilis BSB3 strain or PBS twice a day for 2days. Half of the rats of each group were exposed to heat stress (45 degrees C, relative humidity 55% for 25min), while the remaining rats were placed at identical conditions but at 25 degrees C. Bacterial translocation, histological changes in the intestine, cytokines profile, serum lipopolysaccharide level (LPS), as well as vesiculation of erythrocytes were analysed and compared between groups. Adverse effects of heat stress (morphological changes in intestine, bacterial translocation, elevated levels of LPS and IL-10, increased vesiculation of erythrocytes) were observed only in rats not protected with B.subtilis strain and exposed to heat. All registered parameters in rats pretreated with bacilli and exposed to heat were similar to control groups. ConclusionsBacillus subtilis BS3B strain was effective in the prevention of complications related to heat stress in rats. Significance and Impact of the StudyThis work will contribute towards better understanding of probiotics' mechanisms of action and will bring new approaches to characterize and use of beneficial bacteria.