66 MICROBIOLOGY 



in the sterilizer, the door tightly closed, and the gas lit. 

 When the desired temperature is attained it is held there by 

 an automatic gas regulator or by adjusting the amount of 

 .gas fed to the name. Ordinarily this high temperature is 

 maintained for an hour. This method is used very largely 

 lor sterilizing glassware. It is sometimes used for steriliz- 

 ing instruments and other articles. 



2. Moist heat. In the presence of moisture bacteria 

 ^re destroyed at a much lower temperature than they are in 

 dry air. Moist heat is applied by the use of the water bath. 

 Articles to be sterilized are placed directly in the water, or 

 if liquid, in retainers set in the water, such as glass tubes, 

 or flasks; or by the application of streaming steam; or the 

 use of steam under pressure. 



(a) Water bath. Boiling in water destroys very 

 -quickly the vegetative form of all bacteria. Certain spores 

 ;such as those of Bacterium anthracis and Bacillus siibtili* re- 

 -quire boiling for a considerable length of time for their de- 

 .-struction. If articles that are to be sterilized by boiling in 

 , water bath are not completely submerged in the water, 

 the water bath should be tightly closed. As most of the non- 

 ;spore bearing bacteria are killed at a temperature much 

 below the boiling point, it is possible to sterilize substances 

 at a temperature of 60 C. if they do not contain resist- 

 ing spore bearing organisms. This method is often used in 

 sterilizing substances that do not contain spore-bearing or- 

 ganisms and which would be injured if they were heated to 

 the boiling point. Most non-spore bearing bacteria are 

 killed when held at a temperature of 60 C. for 20 minutes 

 in the presence of moisture. Some of the pathogenic forms, 

 like streptococci, are killed at a lower temperature. 



(b) Streaming steam. Articles to be sterilized by 

 this method are placed in some receptacle into which steam 

 may flow. The Arnold sterilizer is the most common form 

 of apparatus in which to apply steam. The temperature 

 ranges from 98 to 102C. This will kill all non-spore bear- 

 ing bacteria in from 10 to 20 minutes. It is used in the lab- 



