162 CULTURE MEDIA 



medium of many advantages. It is solid at temperatm-es 

 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 fea- 

 ture. The gelatin colonies of many bacteria are very charac- 

 teristic 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, to broth, gently warming until the gelatin is 

 dissolved, standardizing as for broth, filtering and sterilizing. 

 It is usually cleared before filtering by stirring into the gela- 

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

 each 1000 c.c, 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 filtiered 

 boiling hot, but is more conveniently done through absorbent 

 cotton, wet with boiling water. 



Or, the gelatin may be added to meat juice before it is 

 boiled, then this is heated to about body temperature (not 

 too hot, or the proteins will be coagulated too soon) until 

 the gelatin is dissolved. Then the material is standardized 

 and thoroughly boiled and filtered. The proteins of the 

 meat juice coagulate and thus clear the medium without 

 the addition of egg white. Commercial gelatin is markedly 

 acid from the method of manufacture, hence the medivun 

 requires careful titration, even when made from a standard- 

 ized broth. 



Gelatin should be sterilized by discontinuous heating at 

 100° on three successive days, because long boiling or heat- 

 ing above 100° tends to hydrolyze the gelatin into gelatin pro- 

 teose and peptone and it will not gelatinize on cooling. It 

 may be heated in the autoclave for ten to fifteen minutes at 

 10 pounds' pressure and sometimes not be hydrolyzed, but 



