40 LABORATORY BACTERIOLOGY 



same number and kinds of tubes containing lactose, and the 

 same 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. 



54. Preparation of potato for a culture medium. Select 3 

 medium-sized potatoes, thoroughly wash and rinse in boiled 

 water, and cut out, with a cutter made for this purpose, cyhn- 

 ders from 3 to 4. cm. long (oblong rectangular pieces cut with 

 a knife will do quite as well). Ordinarily 2 cylinders can be 

 cut from each potato. These can be cut obhquely, giving 2 

 pieces each. All of the skin must be removed. Wash the 

 potato cylinders in cold running water for 5 minutes (a 

 longer time is preferable), place them in test tubes of the 

 proper size (large or small according to size of cutter used), 

 and add about i cc. of water to each tube. Sterilize them 

 by discontinuous boihng or steaming for 20 minutes each day 

 for 3 consecutive days. Wipe, label, and store in locker. 



55. Preparation of milk for a culture medium. It is better 

 that the cream be removed from the milk before it is used. 

 To do this the fresh milk (about 100 cc.) is placed in a beaker 

 and set in the ice box for from 10 to 15 hours. Then care- 

 fully remove the cream. It is well to filter the milk through 

 a thin layer of absorbent cotton to remove any masses of 

 cream that may be left. The reaction should be tested, and 

 if strongly acid, should be rejected or made 1.5% acid to 

 phenol-phthalein by the addition of n/io sodium hydrate. 

 Distribute the skimmed milk in small test tubes (5 cc. in each) 

 and sterilize by discontinuous steaming in the same manner and 

 for the same length of time as the potatoes. Wipe, label, and 

 store the tubes in locker. 



56. Preparation of litmus milk for a culture medium. This 

 is prepared the same as the milk medium, with the addition of 

 enough of an aqueous solution of litmus to impart a decidedly 



