Background & Objectives: Escherishia coli O157:H7 is one of the most important diarrhea causing agents in developing countries . Using antibiotics cause adverse effects as promoting emergence of antibiotic resistance, fading the microflora of intestine and enhancement of verotoxin (VTEC) production by this bacterium. So, a modern treatment protocol is needed for treatment of infections caused by this bacterium. In this study, Lactobacillus casei (L. casei) was isolated from yogurt and antibacterial effects of bacterial cell debris and its culture supernatant were tested against E.coli O157:H7.

  Methods: Several different samples of yoghurt were cultured in MRS agar in anaerobic conditions at 37 ºC. L. casei was identified by common microbiological and molecular methods. Antimicrobial effects of bacterial cell debris and its culture supernatant were tested against E. coli O157:H7 by using Agar Well Diffusion (AWD) and Broth macrodilution methods. In addition, standard growth curves of pathogenic bacterium and L. casei were obtained by turbidometery and colony count procedures. The MIC (Minimum Inhibitory Concentration) and MBC (Minimum Bactericidal Concentration) of supernatant originated from culture of L. casei were determinded. The stability of antimicrobial effects of the supernatant in different conditions of pH and temperature were studied.

  Results: Lactobacillus casei was isolated from two different samples of yoghurts, and confirmed by phenotypic and genotypic methods. The results showed that antimicrobial effects of culture supernatant were stable at 56, 70, 80^�and 100 ºC for 30 and 60 minutes. Furthermore, they were stable in pH of 3, 7 and 10. The MIC and MBC of supernatants were 1:16 and 1:8 respectively.

  Conclusion: According to the results of this study, culture supernatant of L. casei can be used as a biological preservative in food industries. Also due to antimicrobial effect of L. casei, it can be used in treatment of diseases associated with E. coli O157:H7.