CULTURAL METHODS 77 



is prepared by the action of the enzyme pepsin of the gastric 

 juice upon proteins. Peptones contain mixtures in varying 

 proportions of albumoses, peptids, and amino acids. Those 

 in which digestion is most complete, that is, those which 

 have the highest percentage of amino acid are frequently 

 most useful in stimulating bacterial growth. However, 

 they cannot be used for certain purposes. In the manu- 

 facture of diphtheria toxin, for example, apparently the 

 more complex peptones and albumoses are required. This 

 variation in composition of peptones from different manu- 

 facturers and from different lots should be borne in mind 

 in making comparisons in the growth of bacteria grown on 

 media containing these peptones. 



Sugars and glycerin are frequently added to broth or 

 bouillon in studying the fermentative activities of many 

 species of bacteria, that is, in the study both of acid and 

 of gas production. 



Dunham's solution or peptone water is a solution of pep- 

 tone in water without the addition of meat infusion or beef 

 extract. When the peptone used is of the right composi- 

 tion, that is, when it contains the amino acid tryptophane 

 it may be used for testing the ability of microorganisms to 

 produce the compound indol. 



Milk serves as a satisfactory medium for the growth of 

 many species of bacteria. Usually the fresh separated milk 

 is used ; whole milk may be kept in an ice chest for twenty- 

 four hours and the milk pipetted from below the cream 

 layer. 



Sterile defibrinated blood or blood serum alone, mixed 

 with sterile broth, are frequently used in media, particu- 

 larly when it is desired to grow certain of the pathogenic 

 or disease-producing bacteria. Inasmuch as it is not prac- 

 ticable to sterilize serum without coagulation after removal 

 from the body, precautions are taken to insure sterility and 



