16 Agricultural Bacteriology. 



kept so. For this purpose the various media are placed 

 in glass vessels, such as flasks and tubes, which are 

 stoppered with cotton-wool. This cotton plug allows: 

 the air to pass freely in and out of the vessel, but re- 

 moves all dust and bacteria that % the air may contain. 

 It serves to prevent the bacteria from entering tlje ves- 

 sel from the outside as effectively as though the vessel 

 were sealed air tight. 



Sterilization. After the media is thus protected from 

 future contamination, the contained bacteria are killed 1 

 by heating the media to the boiling point for a short 

 time on each of three successive days. The vegetating 

 cells are easily killed but the spores are not destroyed 

 by the first heating. If the media is stored at ordi- 

 nary temperatures, many of the spores will germinate 

 and form cells before the second heating. The remain- 

 ing spores will usually sprout by the third day and are 

 then destroyed. A heating treatment applied as pre- 

 scribed will usually render any food medium "sterile,'' 

 i. e., absolutely free from all living bacteria or their 

 spores. When so treated it -will keep for an indefinite 

 period if protected from drying. 



The vegetating cells and also the spores may be killed 

 at one heating but the exposure must be very prolonged 

 or it must be made in a closed chamber in which steam 

 is generated. In this latter condition the temperature 

 can be raised quickly to a point considerably higher 

 than the boiling point so that the spores will be killed 

 in a few minutes. This same process is used in the 

 canning of vegetables, such as corn and peas. All such 

 foods must be sterile or they will soon spoil. 



The containers used to hold the media are usually of 

 glass and are rendered free from all living bacteria by 



