NUTRIENT MEDIA 317 



In place of the meat infusion as prepared above, meat extract 

 may be used in the ratio of 4 to 5 gm. per liter of water. 



Bouillon is prepared by adding i liter of water to i pound 

 of chopped lean beef. This is cooked for half an hour, then 

 filtered and neutralized with carbonate of soda, then again 

 boiled for an hour to precipitate albuminoids. After a final 

 filtering the bouillon is poured into flasks or test-tubes and 

 sterilized. 



Infusions of hay and dried fruits may also be used for nutrient 

 media. A hay infusion for the growth of Bacillus subtilis may 

 be prepared as follows: Chopped hay is placed in a beaker and 

 barely covered with well-water; this is kept in an incubator 

 at a temperature of 36° C. for four hours, after which time the 

 extract is poured off and diluted, if necessary, to a specific 

 gravity of about 1.004. The extract is now poured into a flask 

 which, having been closed with a cotton plug, is placed in a 

 steam sterilizer and subjected to a gentle evolution of steam 

 for about an hour. The flask is then placed in an incubator 

 at 36° C. for a day or two, after which time a film produced by 

 colonies of Bacillus subtilis will have formed over the surface 

 of the extract. The spores of this bacterium are particularly 

 resistant to heat, and for this reason while the spores of other 

 bacteria are killed by the process of steaming, those of Bacillus 

 subtilis still retain their vitality. 



Solid culture media may be prepared by adding to any of 

 the fluid culture media a sufficient amount of a gelatinous 

 substance to keep the mixture from liquefying at the tempera- 

 ture of the laboratory, or, if desired, at the higher temperature 

 of an incubator. One of the most used of the solid media is 

 prepared by adding to the peptonized infusion o£ meat, as 

 above described, 10 per cent, of the best French gelatine. The 

 gelatine may be increased up to twice this amount, as the tem- 

 perature may require. One hundred grams of gelatine is allowed 

 to soak, in i liter of the meat infusion until the gelatine becomes 

 swollen, and then a gentle heat is applied until the gelatine is 

 completely dissolved. After the gelatine is dissolved the solu- 



