GELATIN CULTURE MEDIUM 161 



Bacillus coll. This mixture is then boiled to kill the Bacillus 

 coli, retitrated and then 1 per cent, by weight of required 

 sugar is added. Dextrose, saccharose and lactose are the 

 most used, though many others are used for special purposes. 

 After the sugar is added the medium must be sterilized by 

 discontinuous heating at 100° 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 spe- 

 cial 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 sh6uld be boiled and filtered hot 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 

 to 0.8 per cent. acid. The milk should be sterilized discon- 

 tinue usly to avoid splitting up the lactose as well as action 

 on the casein and calcium phosphate. 



Litmus Milk. — Litmus milk is milk as above to which 

 litmus has been added as an acid production indicator. The 

 milk should show blue when the litmus is added or be made 

 to by the addition of normal NaOH solution. It should be 

 sterilized discontinuously. Frequently on heating litmus 

 milk the blue color disappears due to a reduction of the 

 litmus. This blue color will reappear on shaking with air 

 or on standing several days, due to absorption of and 

 oxidation of the reduced litmus, provided the heating has 

 produced no other change in the milk, as proper heating will 

 not. 



Gelatin Culture Medium. — Gelatin to the extent of 10 to 

 15 per cent, is frequently added to broth and gives a culture 

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