94 



PATHOGENIC BACTERIA. 



kill many spores, so that the exposure of culture-media 

 to streaming steam is of little use unless applied in 

 a systematic manner intermittent sterilization based 

 upon a knowledge of sporulation. 



In carrying out the intermittent sterilization the cul- 

 ture-medium is exposed for fifteen minutes to the passage 

 of streaming steam in the apparatus or to some tem- 

 perature judged to be sufficiently high, so that the bac- 

 teria contained in it are killed. As the spores remain 

 uninjured, the medium is stood aside in a cool place for 

 twenty-four hours, and the spores allowed to develop into 

 perfect bacteria. 



When the twenty-four hours have passed the culture- 

 medium is again placed in the apparatus and exposed to 



the same temperature, 

 until these newly-devel- 

 oped bacteria are also 

 killed. Eventually, 

 the process is repeated 

 a third time, lest a few 

 spores remain alive and 

 capable of spoiling the 

 material. When prop- 

 erly sterilized in this 

 way, culture- media will 

 remain free from con- 

 tamination until time 

 and evaporation cause 

 them to dry up. If her- 

 metically sealed, a ster- 

 ile medium will keep 

 indefinitely. 



If it should be neces- 

 sary to sterilize culture- 



FIG. n.-Autoclave for rapid sterilization media ^ Q not wait _ 



by superheated steam under pressure. 



ing the three days re- 

 quired by the intermittent method, it may be done by 

 superheated steam in an autoclave (Fig. n). Here under 



