56 



STERILIZATION OF CULTURE MEDIA. 



which answers the purpose admirably, and which has the advantage 

 of getting up steam very quickly and also of using comparatively 

 little gas. 



The use of steam under pressure, by which higher temperatures 

 are obtained, requires a more expensive apparatus, made on the 

 principle of Papin's digester. The form manufactured by Muncke 

 is one of the best. This is shown in Fig. 27. It is provided with a 

 pressure gauge and a safety valve. A single sterilization in this ap- 

 paratus, at a temperature of 115 C., for half an hour, will usually 



M 



FIG. 26. 



FIG. 27. 



suffice, and for liquid culture media or for agar-agar this method is 

 entirely satisfactory ; but a gelatin medium which is exposed to this 

 temperature loses its property of forming a jelly at 20 to 22 C., and 

 consequently its value as a solid culture medium. In practice the 

 simpler form of apparatus in which streaming steam is used will be 

 found ^to answer every requirement. To insure sterilization with 

 this it is customary to resort to discontinuous heating, as heretofore 

 described. The standard flesh-peptone-gelatin medium should, as 

 a rule, be subjected to a temperature of 100 C. for ten minutes, at 

 intervals of twenty-four hours, four days in succession. Bouillon, 

 flesh infusions, and agar-agar jelly may be steamed for an hour at a 

 time two or three days in succession. 



