34 LABORATORY BACTERIOLOGY 



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. 



61. 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, 

 cylinders 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 obliquely, 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) and place them in test tubes of the 

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

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

 by discontinuous boiling or steaming for 20 minutes each day 

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



52. Preparation of Milk for a Culture Medium. — Place about 

 100 c.c. of fresh milk in a beaker in the ice box and allow it to 

 stand for from 10 to 15 hours. Then carefully remove the 

 cream. It is well to filter the milk through a thin layer of 

 absorbent cotton to remove any masses of cream. The reac- 

 tion should be tested, and if strongly acid it should be rejected 

 or made 1.5% acid to phenolphthalein by the addition of 

 n/ 10 sodium hydratp. Distribute the skimmed milk in small 

 test tubes (7 c.c. in e^ch) and sterilize by discontinuous steam- 

 ing in the same manner and for the same length of time as the 

 potatoes. Wipe, labej, and store the tubes in locker. 



53. Preparation of Litmus Milk for a Culture Medium. — This 

 is prepared the same ^s the milk medium with the addition of 

 enough of an aqueous solution of litmus to impart a decidedly 

 blue color to the milk. Sterilize, wipe, label, and store the 

 same as the milk. The litmus solution will be furnished. 



54. Preparation of Glucose (Grape Sugar) Agar. — Prepare 



