12 



A MANUAL OF BACTERIOLOGY 



SECTION III 



STERILIZATION 



The air in few places is free from bacteria. At times the 

 number of germs in the air is quite large; if they fall upon suit- 

 able media, they soon begin to grow and to cause more or less 

 extensive changes in the composition of the media. Therefore 

 some method of preserving the media from change is necessary. 

 Moreover, for exact work it is generally imperative that we have 

 none other than the desired species under cultivation, that is, a 

 pure culture. 



Hence the necessity arises that the culture medium should not 

 only be cleared of all germ life before using, but that foreign 

 germs should be prevented from entering after the sterile condi- 

 tion is once attained. The operations of sterilization are directed 

 to this end. Heat is the best and most generally applicable method 

 we have for sterilizing culture media. Dry heat and moist heat 

 are both employed, but these differ somewhat in efficiency. 



Exercise 11. Sterilization of Culture Media with Moist Heat in the 

 Arnold Type of Sterilizer 



The most efficient type of sterilizer for work at the tempera- 

 ture of boiling water is the Arnold sterilizer, which has super- 

 seded the old Koch steam chest. 

 The Arnold type of sterilizer 

 is so constructed that only the 

 water in the false bottom needs 

 to be boiled to furnish steam, 

 and the steam circulates in the 

 sterilizing chamber (Fig. 9). 

 The steam which escapes from 

 the top of the chamber is con- 

 densed between the two walls 

 and drips back into the pan.' 

 There should be a thermometer 

 in the roof of the sterilizing 



Fig. 9. Arnold sterilizer 



The false bottom holds only a thin layer 

 of water, which can be quickly boiled 



