GELATIN AND POTATO MEDIA. 161 



tubes, which have been subjected to sterilization, should 

 shov a light yellow tinge. 



Steam Sterilization. 



When the gelatin is filled into tubes, these must be 

 again subjected to sterilization. If this is not done, or 

 done improperly, bacteria will promptly develop in the 

 gelatin and render it worthless. Such instances of "spon- 

 taneous generation " are the result of insufficient exposure 

 to steam-heat. As indicated above, cultural media, such as 

 gelatin, are never sterilized in the dry-heat oven. The de- 

 struction of all the living forms that may be present, is 

 accomplished by the aid of moist heat. 



It will be remembered that bacteria may exist in two 

 stages — the actively vegetating or growing condition, and 

 the resting or spore condition. When in the vegetating 

 state, these organisms, as a rule, are readily destroyed by 

 moist heat. A temperature of 65-70° may do this in 10 

 or 15 minutes, whereas steam-heat (100°) may be said to de- 

 stroy the growing cell instantaneously. On the other hand, 

 the spore, or seed, is exceedingly resistant to destruction. 

 Moist heat of 65-70° has little or no action, and, even 

 steam-heat may require some time. In order to sterilizQ 

 nutrient media it is customary, therefore, to employ steam- 

 heat. They may be sterilized by one prolonged exposure 

 to steam at 100°. Thus, the gelatin tubes might be steril- 

 ized by steaming for one hour. This procedure, indeed, is 

 sometimes resorted to when it is desired to employ the me- 

 dium on the same day on which it is prepared. 



Prolonged steaming tends to alter a medium; it may 

 render the latter acid, and, in the case of gelatin, may soften 

 this to such an extent that it will not congeal on subsequent 

 cooling. By taking advantage of the difference in the resist- 

 ance offered by the vegetating and spore forms, it is possible 



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