M17 Agar with Glycerophosphate, 500 g

M17 media are based on the formulation described by Terzaghi and Sandine (1) for the cultivation and enumeration of lactic Streptococci and their bacteriophages. It is possible to study plaque morphology and lysogeny. M17 Agar is recommended by the International Dairy Federation (5) for selective enumeration of Streptococcus thermophilus from yogurt. M17 Agar is recommended by APHA for the cultivation of lactic Streptococci (6). Lactic Streptococci are nutritionally fastidious and require complex media for optimal growth (2,3). Disodium glycerophosphate maintains the pH above 5.7. The maintenance of pH is very important as lower pH results in injury and reduced recovery of lactic Streptococci. Glycerophosphate does not form precipitate with calcium which is needed for the plaque assay of lactic bacteriophages. Papaic digest of soyabean meal, yeast extract, beef extract and biopeptone provide carbonaceous, nitrogenous compounds, vitamin B complex and other essential growth factors. Lactose is the fermentable carbohydrate and ascorbic acid is stimulatory for the growth of lactic Streptococci. Magnesium sulphate provides essential ions to the organisms. Disodium- #-glycerophosphate maintains the pH above 5.7. The maintenance of pH is very important as lower pH results in injury and reduced recovery of lactic Streptococci. Disodium glycerophosphate suppresses Lactobacillus bulgaricus @ . Shankar and Davies (4) reported isolation and enumeration of Streptococcus thermophilus from yogurt. M17 Agar is also recommended by the International Dairy Federation (5) for selective enumeration of Streptococcus thermophilus from yogurt. It is also suitable for cultivation and maintenance of starter cultures for cheese and yogurt manufacturing. This medium helps in detecting streptococcal mutants that are lactose non-fermenters. Suggested technique to enumerate streptococci is to seed in mass or by stabbing with agar, melted and cooled to 50-55°C, and incubating them at 42°C for 24 hours period. With these conditions, all the colonies might be streptococci. Longer incubation periods or lower temperatures may cause morphological changes in the colonies, which hinders in the recognition of the colonies. Lactose-positive colonies of streptococci are visible after 15 hours and after 5 days they may reach a diameter of about 3-4 mm, whereas those of lactose-negative are 1 mm in diameter. Bacteriophages presence is observed by appearance of characteristic plaques over the bacterial growth. 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. International Dairy Federation, 1981, Joint IDF/ISO/AOAC Group E44. 6. Downes F. P. and Ito K. (Eds.), 2001, Compendium of Methods for Microbiological Examination of Food, 4th Ed., APHA, Washington, D.C.