CULTURE MEDIA 69 



In the preparation of bouillon, one of the most important 

 things to be considered is its neutralization. As a general rule 

 the bacteria require an alkaline rather than an acid reaction, 

 and the medium must, therefore, be made either neutral or 

 very slightly alkaline. The indicators suggested in the 

 various textbooks have been red and blue litmus paper, tu- 

 meric paper, and phenolphthalein. Some organic substances 

 are both acid and alkaline to certain indicators fresh milk, 

 for instance, will turn red litmus paper blue and blue litmus 

 paper red. Other substances, particularly the phosphates, 

 may be neutral to one indicator and acid or alkaline to another. 

 These substances thus give a double or amphoteric reaction. 

 It has been determined by Schultz, and others later, that 

 phenolphthalein is least likely to give ambiguous results, and 

 it has largely replaced all other indicators in this work. It 

 must be understood, however, that it reacts to the carbonates, 

 such as carbon dioxide, and media should be neutralized with 

 this indicator only after being boiled and while still hot. It is 

 also true that the alkaline point with phenolphthalein is re- 

 moved some distance from the neutral point of litmus, so that 

 when a medium is neutralized by means of phenolphthalein it 

 is more alkaline than the bacteria need, and it is necessary on 

 that account, when media is neutralized in this way, to add a 

 certain amount of normal acid to the medium to bring it back 

 to a point most suitable to the bacteria. When a medium is 

 just slightly alkaline to phenolphthalein, it is represented as 

 having a reaction of o. Any alkaline added in addition to this 



