CULTUKE METHODS 23 



Exercise 28. Inoculation of Cultures on Liquid Media 



Bouillon, milk, and the various liquid media are inoculated 

 by introducing the inoculum upon a platinum needle. Transfers 

 from one liquid to another are made with the platinum loop. 

 The liquid media may be used in test tubes or in small flasks 

 of about 250 cc. capacity. 



Exercise 29. Preparation of Milk for Use as a Culture Medium 



Strictly fresh milk must be procured. The cream should be 

 removed either by a centrifugal separator or by shaking out with 

 ether. The milk is placed in test tubes or flasks, as desired, and 

 sterilized in the Arnold sterilizer. Prolonged heating should 

 be avoided, else the proteins will be wholly coagulated. It is 

 well to heat for fifteen minutes on four successive days, then 

 discard such tubes as show, after a three-day incubation, any 

 sign of bacterial growth. 



Exercise 30. Preparation of Litmus Milk 



This useful medium is prepared by adding aqueous litmus 

 solution to fresh milk. Soak 50 g. of dry litmus cubes for twenty- 

 four hours in 250 cc. of distilled water. After filtration through 

 filter paper, enough of this solution is added to the fresh milk 

 to give it a strong lavender color. Sterilize as directed for milk 

 cultures. 



Exercise 31. Preparation of Litmus Whey 



1. Precipitate the casein of fresh milk by rennet extract. 

 Neutralize the filtered whey with 4 per cent citric acid and heat 

 on the steam bath for thirty minutes. 



2. Filter and add litmus solution until a strong lavender color 

 is obtained. Sterilize. 



Exercise 32. Preparation of Whey Agar 



1. Precipitate the casein of fresh milk by adding a few drops 

 of acetic acid to boiling milk. Filter. 



2. Neutralize the whey with sodium hydroxide or bring to 

 1 per cent acid if desired. 



