STERILIZATION OF CULTURE MEDIA. 55 



It is always advisable to test the sterilization of culture material 

 before making use of it. This is done by placing it for a few days 

 in an incubating oven at 30 to 35 C. If a considerable quantity of 

 material in test tubes has been prepared at one time, it will be suffi- 

 cient to put a few tubes in the incubating oven to test sterilization. 



Failure to make this test often leads to serious complications in 

 experimental investigations. A laboratory sometimes becomes in- 

 fected with resistant spores, which are not all destroyed by the usual 

 methods of sterilization, and these may not develop until some time 

 has elapsed after the supposed sterilization. 



Sterilization of Blood Serum. Blood serum which has been 

 collected in test tubes or small flasks, as heretofore directed, is 



FIG. 28. 



sterilized in a water bath at 60 C. (140 F.) by the method of dis- 

 continuous heating. It is usually left in the hot-water bath for 

 about an hour, and this is repeated, at intervals of twenty-four hours, 

 for five to seven days. This rather tedious process may be avoided 

 by collecting the serum in the first instance with proper precautions 

 to prevent it from becoming contaminated with atmospheric organ- 

 isms. A special apparatus was devised by Koch for sterilizing blood 

 serum, but an improvised hot-water bath which is regulated to a 

 temperature of 60 C. by an automatic thermo-regulator will answer 

 the purpose. After being sterilized the serum is solidified by careful 

 exposure to a temperature of about 68 C., which causes it to co- 

 agulate, forming a transparent, jelly-like mass. When coagulated 

 at a higher temperature it becomes opaque. The time required for 

 this operation varies from half an hour to an hour, and it is best to 

 remove the tubes from the receptacle in which they are exposed to 



