CULTURE MEDIA 73 



Liquefiable Solid Media 



Gelatin. Gelatin culture medium is made from bouillon 

 by the addition of gelatin. Gelatin is made from connective 

 tissue, extracted either at ordinary pressure or in the diges- 

 tor (autoclave). Its physical characteristics that are of 

 special interest to the bacteriologist are its melting and solidi- 

 fying points. It is necessary that a gelatin should be used 

 which in ordinary percentages will not interfere with bacterial 

 growth, and will remain solid at the temperature of a warm 

 room. Chemically its most important feature is its acidity, 

 and the higher grades which have a low acidity, compara- 

 tively, should be used. From 10 to 18 per cent of gelatin 

 is added to bouillon in the preparation of this medium. The 

 preparation of gelatin offers no difficulties beyond that of 

 bouillon except that it is necessary to exercise great care in 

 neutralization and to filter it while hot. For the filtration 

 of gelatin and the other liquefiable solid media, wet absorb- 

 ent cotton can be used to advantage, although it is possible 

 to filter the properly prepared medium through filter paper. 

 It is best sterilized in streaming steam on three consecutive 

 days ; but it may be sterilized in an autoclave at a pressure not 

 to exceed ten pounds for fifteen minutes. A fact that must 

 always be borne in mind in preparing gelatin is that its ge- 

 latinizing power is injured by prolonged heating during the 

 process of preparation or sterilization, and is lost immediately 

 when heated to 140 C. in the autoclave. 



