SUGAR BROTHS 173 



Glycerin Broth.— Glycerin broth is made by adding 4 to 

 6 per cent of glycerin to the broth just previous to the 

 sterilization. The glycerin serves as a source of carbon to 

 certain bacteria which will not grow in the ordinary broth— 

 as Mycobacterium, tuberculosis. 



Sugar Broths.— Sugar broths are used for determining 

 the action of bacteria on these carbohydrates, since this is a 

 valuable means of differentiating certain forms, especially 

 those from the intestinal tract. Broth free from sugar must 

 first be made. This is done by adding to broth prepared as 

 already described, a culture of some sugar-destroying organism 

 {Escherichia coli is ordinarily used), and then allowing the 

 organism to grow in the raw broth at body temperature for 

 twenty-four hours. Any carbohydrate in the broth is 

 destroyed by the Escherichia coli. This mixture is then boiled 

 to kill the Escherichia coli, restandardized and then 1 per cent 

 by Aveight of required sugar is added. Dextrose, saccharose 

 and lactose are the most used, though many others are used 

 for special purposes. iVfter the sugar is added the medium 

 must be sterilized hy discontinuous heating dit 100° C for three or 

 four successive days, because long boiling or heating in the 

 autoclave splits up the di- and polysaccharids into simpler 

 sugars and may even convert the simple sugars (dextrose) 

 into acid. 



Various other modified broths are frequently used for 

 special purposes but need not be discussed here. 



Dunham's Peptone Solution, frequently used to determine 

 indol production, is a solution of 1 per cent of peptone and 

 0.5 per cent of salt in tap water. It does not need to be 

 titrated, but should be boiled and filtered into tubes or 

 flasks and sterilized. 



Nitrate Broth.— Nitrate broth for determining nitrate 

 reduction is 1 per cent of peptone, 0.2 per cent of C. P. 

 potassium nitrate dissolved in distilled water and sterilized. 



Milk.— Milk is a natural culture medium much used. It 

 should be fresh and thoroughly skimmed, best by a separator 

 or centrifuge to get rid of the fat. If the milk is not fresh, 

 it should be titrated as for broth and the reaction adjusted. 

 The milk should be sterilized discontinuously to avoid split- 



