Abstract
Problem statement: Lactic Acid Bacteria (LAB) is characterized by fastidious nutritional requirements leading to the need for complex and rich media to allow growth. Owing to their complex requirements which also varied significantly with species, the formulation of selective media for LAB is difficult. Approach: A culture medium for LAB was previously developed based on deproteinated whey supplemented with yeast autolysate and de-lipidated egg yolk. Based on this previously developed medium, some selective media were formulated and their potential for LAB selection and enumeration was examined. Results: In this study, it was considered as a basis to define a selective medium, by adding a pH indicator and some inhibitory compounds to select the LAB flora of various dairy products including milk, disinfected or not, fermented milk and yoghurt. The addition of sodium azide to inhibit the contamination flora, including Gram negative floraand purple bromocresol allowed a direct selection of the Gram + and lactose + flora. An acidic pH (5.0) can also be helpfully considered after protein hydrolysis to avoid protein denaturation. Conclusion: To complete this study, additional work is needed concerning the improvement of culture medium selectivity by considering other inhibitors, of the Gram negative flora (like lithium chloride for instance), of fungi (like actidione), of the Gram positive flora (excepting the LAB flora) and an association of these inhibitors.
Highlights
Lactic Acid Bacteria (LAB) are characterized by fastidious nutritional requirements leading to an important biosynthesis deficiency (Cogan et al, 1997; Loubiere et al, 1996; Monnet and Grippon, 1994; Stackebrandt and Teuber, 1988)
A medium based on deproteinated whey supplemented with yeast autolysate and de-lipidated egg yolk (Djeghri-Hocine et al, 2007) showed an interesting potential for growth of the bacterial flora of the various tested dairy products
The addition of sodium azide, to inhibit the contamination flora, including Gram negative flora, as well as purple bromocresol allowed a direct selection of the Gram + and lactose + flora, characteristics of the majority of the LAB species
Summary
Lactic Acid Bacteria (LAB) are characterized by fastidious nutritional requirements leading to an important biosynthesis deficiency (Cogan et al, 1997; Loubiere et al, 1996; Monnet and Grippon, 1994; Stackebrandt and Teuber, 1988). LAB growth required complex and rich media, containing complex nitrogen sources (peptides), carbon sources, vitamins and minerals (to supply for trace elements) (Amouzou et al, 1985; Juillard et al, 1995; Raccach, 1985). These nutrients should be supplied at optimal concentrations (Benthin and Villadsen, 1996; Cocaign-Bousquet et al, 1995; Desmazeaud, 1994). Several specific and selective culture media are available for the isolation and selection of LAB (De Mann et al, 1960; Rogosa et al, 1951; Talwalkar and Kailasapathy, 2004; Terzaghi and Sandine, 1975; Vinderola and Reinheimer, 1999). A selective media cannot allow the numeration of all LAB genera and species
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