26 GENERAL MICROBIOLOGY 



4. Mix the milk and the azolitmin thoroughly and tube, 

 using approximately 8 c.c. of the litmus milk in each tube. 



Note. Care should be taken to prevent the milk from coming in 

 contact with the top of the tubes, as it will cause the cotton fibers to 

 adhere to the tube. This may be avoided by the use of a " filling 

 funnel." 



5, Sterilize by heating in flowing steam for twenty 

 minutes on four successive days. Milk is difficult to sterilize, 

 owing to the resistant spores which are frequently present. 

 If it is desired to sterilize a larger bulk than in tubes, the 

 time of heating should be lengthened. 



Caution: Overheating tends to change (caramelize) the milk 

 sugar. The color of the azolitmin may also be destroyed. These 

 changes are not desirable. 



EXERCISE 5. PREPARATION OF GLYCERIN POTATO 



A number of chromogenic and pathogenic organisms 

 thrive especially well on media containing glycerin. The 

 manner in which glycerin favors the growth of these organ- 

 isms is not known, but in some instances it seems to be 

 directly utilized for the construction of fat (Bact. tubercu- 

 losis) . 



Apparatus. Large healthy potatoes; cylindrical potato 

 knife, or cork borer; ordinary knife; tumbler; sodium car- 

 bonate, 1 : 1000 solution; glycerin, 5% solution; large 

 sterile test tubes, or Roux potato tubes; absorbent cotton or 

 short glass rod; 1 c.c. pipette; distilled water; apparatus 

 for steam sterilization. 



Method. 1. Carefully clean one or two large potatoes. 



2. By means of a cylindrical potato knife or cork borer, 

 cut cylinders of potato, 4 to 6 cm. long and 1.5 to 1.8 cm. 

 in diameter. With an ordinary knife, halve each cylinder 

 by a diagonal cut so that each piece resembles in shape an 

 agar slant. Remove any portions of the skin on these pieces. 



3. Place in a tumbler and soak in a dilute (1 : 1000) 

 solution of sodium carbonate* for twenty-four hours only. 



* Sodium carbonate is used to neutralize the natural acids of the potato. 



