STERILIZATION 



13 



chamber. Plenty of water should always be supplied to the 

 lower pan before heat is applied. 



Culture media should be steamed for twenty minutes, begin- 

 ning to count the time when the thermometer records 100 C. 

 The media should be steamed on 

 three successive days. Why ? 

 Between steamings the media 

 should be kept at temperatures 

 favorable for spore germination. 

 Why ? This is known as Tyn- 

 clall's method of fractional steril- 

 ization, or as Tyndallization. 



Exercise 12. Sterilization of Cul- 

 ture Media with Moist Heat in 

 the Autoclave 



Both spores and bacteria are 

 killed within half an hour when 

 exposed to moist heat at tem- 

 peratures of 110-120 C. In 

 practice these temperatures are 

 usually obtained by sterilization 

 in a strong, tightly closed cham- 

 ber. The autoclave is the form 

 generally used (Fig. 10). 



The autoclave is first supplied 

 with a small quantity of water, 

 the culture media are put in, 

 and the lid screwed 011 tightly. 

 Heat is applied to the bottom 

 to boil the water supplied. The 

 stopcock at the top of the cham- 

 ber is kept open until all the air 



is expelled and steam escapes from the opening. The chamber 

 may then be assumed to be at a temperature of 100 C'. and 

 to be nearly if not entirely free from air. The stopcock is then 

 closed and pressure developed up to 15 Ib. The autoclave is 



FIG. 10. Autoclave fitted with steam 

 connections 



This type can also be heated by a large 

 gas burner 



