Antibacterial Activity of Probiotic Yoghurt and Soy-Yoghurt against Escherichia coli and Staphylococcus aureus

Introduction: Biopreservation systems in food are becoming increasingly interesting for the industry and consumer. Methods: Yoghurt milk and soymilk samples were inoculated separately with E.coli or S. aureus immediately after adding the starter (bifidobacteria and/or yoghurt culture) to investigate the antimicrobial activity of probiotic yoghurt and soy-yoghurt. Results: Probiotic yoghurt containing Bifidobacterium lactis (Bb-12) and Bifidobacterium longum (Bb-46) exhibited a slight pH drop compared with plain yoghurt (without bifidobacteria) during the refrigerated storage period. Plain yoghurt and probiotic yoghurt containing Bb-12 and Bb-46 inoculated with or without test organisms showed a significant (P<0.05) increase in lactic and acetic acids than the probiotic soy-yoghurt containing Bb-12 and Bb-46 which produces not only lactic and acetic acids but also formic acid. From the initial count of E.coli, % decreases were 96.00, 99.43, 99.14, 97.14 and 98.43%, for the plain yoghurt, probiotic yoghurt containing Bb-12 & Bb-46 and soy-yoghurt containing Bb-12 and Bb-46, respectively. E. coli counts were disappeared in probiotic yoghurt, soy- yoghurt and plain yoghurt after 2, 3 and 5 days of storage, respectively .The decrease percentage for the plain yoghurt, probiotic yoghurt containing Bb-12 and Bb-46 and soy-yoghurt containing Bb-12 & Bb-46 were 85.62, 93.36, 95.58, 93.36 and 95.58 from the initial inoculum level, respectively. The growth of S.aureus was not detected in the probiotic yoghurt containing Bb- l 2 and Bb-46 after the 10th day of storage. Low numbers of S.aureus survived in the plain yoghurt and probiotic soy-yoghurt Bb- 12 and Bb-46, after 15 days of cold storage. Conclusion: According to this data probiotic yoghurt, Soy-yoghurt and their antibacterial metabolites can be used to control pathogenic microorganisms