174 CULTURE MEDIA 



ting up the lactose as well as action on the casein and cal- 

 cium phosphate. 



Litmus Milk.— IXtmn^ 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 O 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 

 medium of many advantages. It is solid at temperatures 

 up to about 25° and fluid above this temperature, a property 

 which is of great advantage in the isolation of bacteria. 

 (See Chapter XVIII.) Further, gelatin is liquefied (that is, 

 digested, converted into gelatin proteose and gelatin pep- 

 tone, which are soluble in water and do not gelatinize) by 

 many bacteria and not by others, a valuable diagnostic 

 feature. The gelatin colonies of many bacteria are very 

 characteristic in appearance, as is the growth of many on 

 gelatin in culture tubes. 



Gelatin medium may be prepared by adding the proper 

 amount of gelatin (10 to 15 per cent by weight) broken into 

 small pieces (powdered gelatin in the same proportion may 

 be used) to broth, gently warming until the gelatin is dis- 

 solved, standardizing as for broth, filtering and sterilizing. 

 It is usually cleared before filtering by stirring into the 

 gelatin solution, cooled to below 60°, the white of an egg for 

 each 1000 cc, and then thoroughly boiling before filtering. 

 The coagulation of the egg albumen entangles the suspended 

 matter so that the gelatin filters perfectly clear, though 

 with a slight yellowish color. The filtering may be done 

 through filter paper if the gelatin is well boiled and filtered 

 boiling hot, but is more conveniently done through absorb- 

 ent cotton, wet with boiling water: 



