40 



PHARMACEUTICAL BACTERIOLOGY. 



discontinuous or fractional method answers the purpose admirably. Place 

 the test-tubes, flasks, and other cotton-plugged containers with culture media, 

 in a steam sterilizer (Arnold steam sterilizer, either board of health or cyhn- 

 drical form; or kitchen vegetable cooker or steamer). The test-tubes are 

 placed in wire baskets (rectangular or cylindrical). These several containers 

 with culture media are exposed to live steam for about thirty minutes, where- 

 upon the flame is turned out, and if convenient the containers are allowed 

 to remain in the sterilizer. Caution must be observed to guard against con- 

 densed steam running into the several containers. The better way is to 



remove the containers and place them in an 

 incubator kept at a temperature of 20° C. In 

 twenty-four hours, or thereabouts, steam is 

 again applied for thirty minutes. This is re- 

 peated a third time on the second day after 

 the first sterilization. The first sterilization 

 presumably kills most of the vegetative cells. 

 During the first interval of twenty-four hours 

 most of the spores present develop into vege- 

 tative cells, which are killecf at the second 

 sterilization. Should any survive the second 

 steaming, they are sure to be killed during the 

 third sterilization. During this time the cotton 

 plugs have not been removed. The media 

 thus fractionally or discontinuously sterilized 

 are now ready for use in making microbic 

 cultures, or they may be set aside for an indef- 

 inite period of time. 



It is, of course, evident that in the above 

 process of sterilization the temperature does 

 Fig. 16.— Autoclave for using not exceed 100° C, and it may be less in cer- 



steam under pressure for pur- . . r , .i- 



poses of sterilization. tam portions of the sterilizer, steamer, or 



cooker, say, 95° to 97° C. In large or well- 

 equipped bacteriological laboratories certain kinds of sterilizations are done 

 by steam under pressure. The apparatus used for this purpose is known as 

 autoclave. It consists of a strong steam cylinder with a screwed-down top 

 safety valve, steam gauge, and thermometer. The articles (media, etc.) to 

 be sterilized are placed inside, the top is securely fastened down, steam is 

 generated until the thermometer registers, say, 120° C. The temperature 

 is kept up to that degree for about five to ten minutes, which is sufficient to 

 destroy all life, including spores. For ordinary purposes the autoclave is 

 not essential. In fact, its use is rather limited. Blood serum, gelatin media, 

 and all media containing carbohydrates, undergo certain chemical changes 



