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  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.

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Background & objectives: Probiotic dairy products as a functional food have a positive effect on the health of consumers that have been confirmed by scientific evidence. Yoghurt is considered as one of the most popular dairy foods in Iran, which the incorporation of the probiotics into the yoghurt matrix can improve their microbiological and sensory properties and subsequently will play a vital role in promoting the health of the community. The aim of this study was to investigate the effect of yeast Kluyveromyces marxianus on the microbial and sensorial properties of probiotic yoghurt during refrigerated storage.
Methods: In this in vitro study, the yeast Kluyveromyces marxianus PTCC=5189 was used to produce probiotic yoghurt and Aspergillus parasiticus PTCC=5018 (IR 63) and Penicillium chrysogenum PTCC=5074 were used as known pathogens and the main spoilage agents in yoghurt. Changes in the number of probiotic yeasts and spoilage species as well as the sensory acceptability of the samples during 28 days of storage at 4°C were evaluated and compared with the control samples.
Results: The population of both species, involved in yoghurt spoilage, during refrigeration were significantly reduced by the presence of Kluyveromyces marxianus (p<0.01). After 28 days of storage, the number of Kluyveromyces marxianus was at the recommended level of the International Dairy Federation with a 7.35 log CFU/g. The sensory evaluation results demonstrated that the control yoghurt samples were more acceptable.
Conclusion: Kluyveromyces marxianus has a significant effect on improving microbiological properties and can be used in the formulation and production of probiotic yoghurt with high storage capability and optimal sensory acceptability.