M17 Agar is a nutritionally rich medium used for the cultivation and enumeration of fastidious lactic streptococci. It is recommended to isolate Streptococcus thermophilus from yogurt and suitable for growing and maintaining starter cultures for cheese and yogurt manufacture. M17 Agar contains sodium glycerophosphate which has sufficient buffering capacity to maintain the pH above 5,7. Actively growing cultures, as these homofermentative organisms produce large amounts of acid and need a good buffer to maintain the pH above 5,7, thereby guaranteeing the growth conditions of the organisms. This maintenance of the pH is important since a lower pH may cause injury and a reduced recovery of lactic streptococci. Soy, meat and casein peptones, as well as beef extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is the source of vitamins, particularly of the B-group. Lactose provides the carbohydrate as an energy source. Sodium glycerophosphate increases the buffering capacity of the medium and maintains the pH. Ascorbic acid stimulates the growth of lactic streptococci. Magnesium sulfate provides essential ions for growth. Bacteriological agar is the solidifying agent.
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