35 



these media may be regarded somewhat as a test, to deter- 

 mine the presence or absence of certain properties. Thus, 

 for example, will the species in hand coagulate the casein in 

 milk, produce gas in media containing saccharose, grow on 

 potato, etc.? The number of these tests which have been 

 used and recognized as important is quite large, but in a 

 short course only those possessed of the more differential 

 value can be tried. In describing a new species, or identi- 

 fying any of the carefully described ones, it is important to 

 know at least some of these cultural peculiarities and bio- 

 chemic properties. For this reason it is necessary to learn 

 the method of preparation and the use of certain of these 

 media. 



In addition to the above, a few species of bacteria require 

 a particular kind or kinds of media for their diagnostic or 

 most differential growth. 



Among these are those of glanders, diphtheria and tuber- 

 culosis. The preparation of these particular media will be 

 considered in connection with the study of the organisms 

 requiring them. 



46. General Directions. Prepare for culture media, 

 5 tubes of potatoes, 5 tubes of milk, 5 tubes of litmus milk, 

 5 tubes of glucose agar, 5 tubes of glycerin agar, 4 fermenta- 

 tion tubes of bouillon containing glucose, 4 containing 

 lactose, and 4 containing saccharose. (The agar and the 

 sugar free bouillon necessary in the work of this exercise 

 will be furnished by the instructor.) 



Read carefully the paragraphs in the text books on the 

 preparation and use of these media. 



47. Preparation of potato for a culture medium. 

 Select medium sized potatoes, thoroughly wash and cut out, 

 with a cutter made for this purpose, a cylinder 3 to 4 cm. 

 long (oblong pieces cut with a knife will do quite as well). 

 Ordinarily 2 cylinders can be cut from each potato. The 

 inclined surface is obtained by cutting out the potato pro- 



