CULTIVATION OF BACTERIA 35 



In media-containing sugars some bacteria ferment the 

 sugars with the production of acid, others with the 

 production of gas and acid, while still others do not fer- 

 ment the sugar at all. In studying the production of 

 acid we can add a little litmus to the medium and note 

 whether the blue color changes to red. The formation 

 of gas is best observed by growing the culture in a fer- 

 mentation-tube. 



Demonstration. The teacher should inoculate various media, 

 both liquid and solid, using organisms growing both at room tem- 

 perature and body heat (e. g. % hay bacillus), and organisms growing 

 only at body heat (e. g., pneumococcus) . Some of the latter variety 

 should be inoculated in duplicate, and the duplicate kept at room 

 temperature, to show that there will be no growth under these 

 conditions. Agar-plate cultures should be made from milk (diluted) 

 and from tap- water. 



Streak-plate cultures may be made from the throat or from teeth 

 scrapings. 



