The World Health Organization's Food Safety Unit has given high priority to study fermentation as a technique for food preparation and preservation because in developing countries one-tenth of under-five children die from dehydration. Loss of water is mainly due to the spreading of diarrhea, and the cause of diarrhea are foods that do not meet the standards of hygiene, the health standard of a food is based on the process and conditions of the raw material, and lactic fermentation of food as a standard process has been known to reduce the risk of growth of foodborne pathogens. In this study, the effect of a probiotic strain of Lactobacillus fermentum on the growth of Escherichia coli O111 during yogurt storage was evaluated. Different conditions were used in this study: concentration of L. fermentum at three levels and E. coli O111 at one level. In probiotic yogurt, the L. fermentum count and E. coli O111 count, pH, acidity, and syneresis were evaluated. The results showed that the total count of E. coli O111 in the control sample was higher than the probiotic samples. Probiotic bacteria also decreased during the storage period. The results indicate that probiotic yogurt had antimicrobial properties during storage. Syneresis characteristics also showed that the control sample had more syneresis than the other samples. The results of this study showed that L. fermentum has antimicrobial potential in dairy products پرونده مقاله