M17 agar is based on the formulation described by Terzaghi and Sandine (1) for the cultivation and enumeration of lactic Streptococci and their bacteriophages.
Lactic Streptococci are nutritionally fastidious and require complex media for optimal growth (2, 3).
Casein enzymic hydrolysate, papaic digest of soyabean meal, yeast extract, meat extract provide carbonaceous, nitrogenous compounds, vitamin B complex and other essential growth factors. Lactose is the fermentable carbohydrate. Ascorbic acid is stimulatory for the growth of lactic Streptococci. Magnesium sulphate provides essential ions to the organisms. Diodium ßglycerophosphate maintains the pH about 5.7 due to its buffering action.
Shankar and Davies (4) reported isolation and enumeration of Streptococcus thermophilus from yoghurt. Disodium glycerophosphate suppresses Lactobacillus bulgaricus. This medium is a standard medium for isolating lactic streptococi (5). M17 Agar is recommended by the International Dairy Federation (6) and ISO Committee (7) for selective enumeration of Streptococcus thermophilus from yoghurt. It is also suitable for cultivation and maintenance of starter cultures for cheese and yoghurt manufacturing.
Storage and Shelf-life:
Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label.
References:
1. Terzaghi B.E. and Sandine W.E., 1975, Appl. Microbiol., 29:807.
2. Anderson A.W. and Elliker P.R., 1953, J. Dairy Sci., 36:161.
3. Reiter B. and Oran J.D., 1962, J. Dairy Res., 29:63.
4. Shankar P.A. and Davies F.L., 1977, Soc. Dairy Technol., 30:28.
5. Downes F.P and Ito k. (Eds), 2001, Compendium of Methods for Microbiological Examination of Food, 4th Ed. APHA, Washington, D.C.
6. International Dairy Federation, 1981, Joint IDF/ISO/AOAC Group E44.
7. International Organization for Standardization, 1985, ISO/DIS: 7889.