48 



CULTURE MEDIA. 



reaction. Certain bacteria grow in this solution without producing 

 any change in its color ; others, which produce an acid reaction, 

 cause it to be decolorized ; others, which produce an alkaline reac- 

 tion of the medium, change the color to green. 



Cooked Potato. Schroter first used cooked potato as a culture 

 medium for certain chromogenic bacteria (1872), and Koch subse- 

 quently called attention to the great value of potato cultures for 

 differentiating species. His plan of preparing potatoes is as follows : 

 Sound potatoes are chosen in which the epidermis is intact. These 

 are thoroughly washed and scrubbed with a brush to remove all 

 dirt. The " eyes" and any bruised or discolored spots are removed 

 with a sharp-pointed knife. They are again thoroughly washed in 

 water, and are then placed for an hour in a bath containing 

 mercuric chloride in the proportion of 1 : 500, to thoroughly disinfect 

 the surface. They are then placed in a steam sterilizer for about 

 three-quarters of an hour, and after an interval of twenty-four hours 



A 



Fro. 80. 



Flo. 21. 



Fio. 22. 



are again steamed for fifteen minutes. It is well to wrap each 

 potato in tissue paper before placing it in the bichloride bath, and to 

 leave it in this protecting envelope until it is placed in the glass dish 



