SPECIAL CULTURE MEDIA 135 



This operation is necessary because earth bacteria often adhere to 

 the outer surface and it is desirable to get rid of their exceedingly 

 resistant spores which can withstand steam sterilization for more than 

 four hours. Cut out the eyes, and, after peeling, cut the potatoes into 

 slices, subsequently to be placed in Petri dishes (see Chapter XII) and 

 into cylinders which are divided into two equal masses to be placed in 

 test-tubes. Cylinders are prepared with an ordinary household apple 

 corer or with a special device, the Ravenel potato cutter. The slices 

 and cylinders are then best washed in running water for several hours 

 to insure partly against subsequent undesirable changes. After wash- 

 ing, the material is placed into the glass receptacles. The tubes first 

 receive a piece of broken glass or some glass wool on which the half 

 cylinder rests, and a little distilled water to supply moisture. The 

 potatoes then undergo fractional or continuous sterilization. In 

 spite of all precautions, potato media frequently dry out and become 

 dark during preservation or later in the incubator. Special points 

 in the use of potatoes are mentioned under anthrax and glanders (see 

 Chapters on these Bacteria). 



Special Culture Media. Certain growing bacteria have definite 

 biologic characteristics, such as the production of indol, etc., and a 

 number of special culture media are used to determine these. Other 

 special media are free from albumins or proteid matter, and contain 

 chemicals of well-known formula? only. Some media of this type are: 



Dunham's Peptone Water. 1. Witte's dry 'pep tone, 10 grams; 

 common salt, 5 grams; mix with 250 c.c. of distilled water, heated to 

 60 C. 



2. Place into a flask and make up with distilled water to 1000 c.c. 



3. Heat to boiling for one-half hour. 



4. Cool and filter. 



5. Distribute to test-tubes and sterilize in the steam sterilizer for 

 twenty minutes on three consecutive days. 



Peptone Rosolic Acid Water. 1. Take Dunham's peptone water, 

 100 c.c., and add 2 c.c. of J per cent, alcoholic solution of rosolic 

 acid (coralline). 



2. Heat for one-half hour. 



3. Filter through filter paper. 



4. Distribute to test-tubes and sterilize as above. 



Nitrate Water. 1. Dissolve 10 grams of peptone in 1000 c.c. of 

 ammonia-free distilled water. 



2. Heat in steam sterilizer for twenty minutes. 



3. Add 1 gram of sodium nitrite (C. P.). 



4. Filter, distribute to tubes, and sterilize as above. 

 Albumin-free Solutions. Pasteur's Solution. 



Ammonium tartrate ......' 1 part 



Saccharose or cane sugar 10 parts 



Yeast-ash 1 part 



Water . . 100 parts 



