54 STERILIZATION OF CULTURE MEDIA. 



to be sterilized. The form shown in Fig. 23 is commonly used in 

 bacteriological laboratories. 



It must be remembered that a much higher temperature is re- 

 quired for the destruction of microorganisms when dry heat is em- 

 ployed than is the case with moist heat. The experiments of Koch 

 and Wolffhiigel (1881) show that a temperature of 120 to 128 C. 

 (348 to 262 F.) is required to destroy the spores of mould fungi, and 

 micrococci or bacilli in the absence of spores. For the spores of ba- 

 cilli a temperature of 140 C. (284 F.), maintained for three hours, 

 was required. 



In practice we usually maintain a temperature of about 150 C. 



FIG. 23. 



(302 F.) for an hour or more; and it is customary to sterilize all 

 test tubes and flasks, which are to be used as receptacles for culture 

 media, in the hot-air sterilizer. This procedure could no doubt, how- 

 ever, be dispensed with in many cases and reliance be placed upon 

 the sterilization of the flask, together with its contents, in the steam 

 sterilizer, especially with such culture media as are not injured by 

 long exposure to a boiling temperature e. g. , bouillon and agar-agar. 

 When we propose to cultivate aerobic bacteria, or such as require 

 oxygen for. their development, a cotton air filter is placed in the 

 mouth of each test tube and flask before it is sterilized in the hot-air 

 oven. This is a loose plug of cotton, pushed into the neck of the 

 flask for an inch or more, and projecting from its mouth for a short 

 distance. These cotton filters should fill the tube completely and 



