140 BACTERIOLOGY 



Litmus-whey Milk. An important differential medium 

 is milk-whey to which litmus tincture has been added. 

 A number of methods for its preparation are in use, but the 

 one employed by Durham seems to be the most satisfactory. 

 Briefly it is as follows: fresh milk, free from antiseptic 

 adulterations, is gently warmed and clotted with essence 

 of rennet. The whey is strained off and the clot hung up 

 to drain in a piece of muslin. The whey, which is somewhat 

 turbid and yellow, is then cautiously neutralized with a 4 

 per cent, citric acid solution, neutral litmus solution being 

 used as the indicator. It is then heated upon a water-bath 

 to 100 C. for about half an hour; thereby nearly the whole 

 of the proteid is coagulated. It is then filtered clear and 

 neutral litmus solution is added until it is of a distinct purple 

 color. If the filtered whey is cloudy, let it stand in a cold 

 place for a day or two and decant off the clear supernatant 

 portion or pass it through a Berkefeld filter. The whey should 

 never be heated above 100 C. or neutralized with mineral 

 acids, otherwise there is a danger of so modifying the milk- 

 sugar present as seriously to impair the usefulness of the 

 medium. When properly prepared, the medium is free from 

 proteid, and contains only water, lactose, the salts of the 

 milk, and a small quantity of a body suggestive of dextrose 

 or galactose. The medium is of great utility in detecting 

 the power of bacteria to cause acid fermentation in a non- 

 proteid medium containing a fermentable sugar; and for 

 observing the variations of this power in closely allied though 

 not identical species. 



Dunham's Peptone Solution. The hiedium known as 

 Dunham's solution is prepared according to the following 

 formula : 



Dried peptone 1.0 part 



Sodium chloride . . . . . . . . . 0.5 part 



Distilled water 100.0 parts 



