CHAPTER VII. 



BACTERIOLOGICAL LABORATORY TECHNIC. 



In order to study bacteria by other methods than the simple 

 examination of their morphology by means of stains, and by the 

 hanging drop, or block method, they must be cultivated either in 

 the bodies of experiment animals, or in culture media artifici- 

 ally prepared. The latter method is the most widely used in 

 laboratories. It is necessary, in order to study bacteria, that the 

 media shall not contain any extraneous bacteria to begin with, 

 and that they shall be cultivated under such conditions that these 

 bacteria cannot reach the media at any time. To accomplish all 

 this, the culture media must be kept in glass vessels, such as test- 

 tubes and flasks that have been sterilized. And, since all animal 

 and vegetable substances, not actually alive, are overwhelmed 

 with a multitude of bacteria, these substances must be sterilized 

 too, in order that the media shall be free from any living organisms. 



Glassware, such as pipettes, Petri dishes, flasks and test-tubes, 

 are sterilized best by dry heat in hot air sterilizers. The apparatus 

 is subjected to a temperature of 150 C. for one hour, or until the 

 cotton plugs are slightly brown. The glassware should be put in 

 wire baskets and the test-tubes should be kept erect. Petri dishes 

 are best sterilized in a wrapping of paper. Flasks and test-tubes 

 are always plugged with raw cotton, which prevents the ingress of 

 bacteria, while air can reach the media through it freely. 



Sterilization of culture media is accomplished in steam sterilizers 

 of two patterns; of these, the autoclave, using steam under pressure, 

 is the most satisfactory and is most generally used at present. 



The baskets containing the culture media are placed in the auto- 



