22 Dairy Bacteriology. 



for twenty-four hours, at temperatures favorable for 

 the germination of the spores and are then again heated. 

 Three such applications on successive days are usually 

 sufficient to free the media from all living germs, since 

 between the heating periods the spores germinate and 

 the resulting vegetative cells are more easily destroyed. 

 The sterile media will keep for an indefinite period in 

 a moist place. 



The media are usually placed in glass containers 

 which may be sterilized before use by heating them in 

 an oven, it being possible to thus secure a much higher 

 temperature than with streaming steam. All glass or 

 metal articles may be sterilized by the use of dry heat 

 but for organic media, to avoid burning, moist heat 

 must be used. 



All kinds of materials may be sterilized by treatment 

 with steam under pressure. An exposure for a few 

 moments at 250 F., a temperature attained with 15 

 pounds steam pressure, will destroy all kinds of bacteria 

 and their spores. This method of sterilization is used 

 in the canning of meats and vegetables and in the prep- 

 aration of evaporated milk. To avoid contamination of 

 the media after sterilization, the flasks and tubes are, 

 after being filled, stoppered with plugs of cotton-wool, 

 which effectually filter out all bacteria and mold spores 

 from the air, and yet allow the air to pass freely in 

 and out of the containers. 



Methods of determining the number of bacteria. 

 The method of determining the number and kinds of 

 bacteria in any substance can be illustrated by the pro- 

 cess as applied to milk. For this purpose the method 

 of procedure is as follows: Sterile gelatin in glass 

 tubes is melted and then cooled until it is barely warm. 



