fa عمومى General پژوهشي Research <span style="font-size:11pt"><span style="line-height:120%"><span style="font-family:Calibri,sans-serif"><span lang="TR" style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">Nowadays, the use of probiotics is considered an alternative solution instead of using antibiotics.</span></span></span> <span lang="TR" style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">This study aimed to investigate the nutritional effect of probiotic <i>Lactobacillus acidophilus</i> on intestinal microbial flora and inhibition of Shiga toxin (Stx2) gene expression in zebrafish exposed <i>to Escherichia coli</i> O157:H7. For this purpose, 600 zebra fish were divided into four groups with three repetitions including feeding with a commercial diet without probiotics (C and T3) and feeding with a diet containing probiotics with a concentration of 1.5&times;10⁸ cfu ml^-1 (T1 and T2). The fish were fed for 24 days. Then T2 and T3 groups were exposed to <i>E. coli</i> O157: H7 for 72 hours. Sampling was done on days 0, 14, 24, and 28. The obtained results showed that feeding with probiotics led to an increase in the growth of lactobacilli in the intestine (<i>p</i><0.05). Also, <i>L. acidophilus</i> decreased the expression of the Stx2 gene in fish exposed to the pathogen (<i>p</i><0.05). Therefore, <i>L. acidophilus</i> used in the diet improved intestinal microbial flora and inhibited toxin production in zebrafish exposed to <i>E. coli</i> O157: H7. This pathogen is very important in terms of prevalence, nutritional role, and the effect of intestinal microbial flora in reducing the pathogenicity <i>of E. coli</i>. In addition, zebrafish is an animal model to investigate the pathogenicity of human pathogens. Therefore, the results obtained can be generalized to humans.</span></span></span><span dir="RTL" lang="AR-SA" style="font-size:12.0pt"><span style="line-height:120%"><span style="font-family:"Times New Roman",serif"></span></span></span></span></span></span><br> &nbsp; Lactobacillus acidophilus, Escherichia coli O157: H7, Shiga toxin, Zebrafish 1 11 http://injvr.com/browse.php?a_code=A-10-1-2&slc_lang=fa&sid=1 P. Salimi No Department of Biology, Roudehen Branch, Islamic Azad University, Roudehen, Iran. R. Kazempoor r.kazempoor@riau.ac.ir Yes Department of Biology, Roudehen Branch, Islamic Azad University, Roudehen, Iran.