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 Liffler.—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 gerins 
after being inoculated with-some particular microédrganism. 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 Bolton, 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. 
