48 CULTURE MEDIA. 



two or three per cent of lactose and enough tincture of litmus to give 

 the culture medium a pale blue color. Colonies of bacteria growing 

 in this medium which cause a fermentation of the lactose, with 

 formation of acid, have a pale pink color, extending to the surround- 

 ing medium. Colonies which do not give rise to acid production 

 are pale blue. Thus, colonies of the colon bacillus would be red and 

 colonies of the typhoid bacillus blue. 



Blood-serum Mixture of Loffler. This consists of three parts 

 blood serum and one part of neutral meat infusion, containing one per 

 cent of glucose. It is sterilized and solidified as directed for blood 

 serum, but a higher temperature is required for coagulation of the 

 mixture than for plain blood serum. 



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 

 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 

 in which it is preserved from contamination by atmospheric germs 

 after being inoculated with some particular microorganism. Just 

 before such inoculation the potato is cut in halves with a sterilized 

 (by heat) table knife. The bacteria to be cultivated are placed upon 

 the cut surface and the potato is preserved in a glass dish (Fig. 20). 



A more convenient method, and one which secures the potato more 

 effectually from atmospheric organisms, is to cut a cylinder, about 

 an inch in diameter, from a sound potato, by means of a tin instru- 

 ment resembling a cork borer or apple corer. This cylinder is cut 

 obliquely into two pieces having the form shown in Fig. 22, and 

 each, piece is placed in a large test tube having a cotton air filter, in 

 which it is sterilized. This method, first employed by Boltoii, has 

 been slightly modified by Roux, who recommends that a receptacle 

 for catching the water which separates during the sterilizing process 

 be formed by making a constriction around the test tube an inch 

 above its lower extremity. This is done by the use of a blowpipe. 



