MEDIA 57 



Ex. 28. The following modifications are frequently needed 

 for special purposes : 



Nutrient Sugar Gelatines. Add to the ordinary gelatine 

 medium of previous exercise before final filtration 



(a) Lactose sugar or lactose, 20 grams, or 



(b) Glucose sugar or glucose, 20 grams. 



Ex. 29. Litmus-Glucose Gelatine. Before sterilization add 

 to a neutral gelatine medium 2 per cent, of glucose and enough 

 Kubel-Tiemann's neutral litmus to colour it a purple tint. 



Colonies producing acids in this medium become surrounded 

 by a pink halo. 



Ex. 30. i. Nutrient Agar. For its preparation see Ex. 20. 



2. Milk-sugar or Lactose Agar. Add to the ingredients 

 mentioned in list (i) Ex. 20 in the preparation of nutrient agar 



Milk-sugar or lactose, 20 grams. 



3. Grape-sugar or Glucose Agar. To nutrient agar add 



Glucose, 20 grams. 



4. Glucose-chalk Agar. To a neutral agar add enough 

 sterilized precipitated chalk to render the medium white and 

 opaque. When grown on plates of this medium, colonies which 

 produce acids dissolve the chalk and make the medium clear. 



5. Litmus-glucose Agar. Before sterilization add to a neutral- 

 ized agar enough Kubel-Tiemann's litmus to give the medium 

 a purple colour. 



Ex. 31. Milk. Take some perfectly fresh milk and pass it 

 through a separator. 



If the milk is acid, make it just neutral to litmus by adding 

 normal caustic soda solution or sodium carbonate solution. 



Place the separated milk in sterilized tubes, and sterilize 

 in steam on three successive days for twenty minutes each time. 

 In order to be quite certain that the milk is free from spores 

 of bacteria, the tubes should be placed in an incubator for 

 twenty-four hours before being inoculated with the bacteria to 

 be studied : if organisms are present curdling or other changes 

 in the milk will usually be noticed. 



