88 METHODS USED IN THE STUDY OF BACTERIA 



pure culture of the lactic acid bacteria. This he did by count- 

 ing under the microscope the number of bacteria in a drop of 

 milk. He then added sufficient sterile water so that any 

 particular drop of the mixture would contain not over a single 

 organism. He then added a drop of this mixture to each of 

 five sterile test tubes of milk. Only one of these tubes coagu- 

 lated, and he naturally inferred that his object was attained 

 and that he had in that one tube of milk a pure culture of the 

 lactic acid bacteria. Naegeli and Fritz afterwards used this 

 method. Pasteur applied these principles to the study of 

 yeast, but he diluted his yeast with air rather than with water. 

 Dry yeast was mixed with powdered gypsum, and this mix- 

 ture was thrown into the air. As the fine dust of the mixture 

 settled, it fell into flasks about the room containing a suitable 

 culture medium and from which the cotton plugs had been 

 removed. Hanson diluted yeast with water and distributed 

 the mixture in flasks. He took a step forward, however, 

 by allowing them to remain undisturbed for a few days, 

 and then rejected as impure those having more than a single 

 speck in a bottle. This method of dilution was carried to the 

 extreme by the French school. While it is possible to get a 

 pure culture in this way, it is quite impossible to isolate all 

 the bacteria occurring in a mixture whose content is known. 

 Use of Liquefiable Solid Media. This was suggested by 

 the use of potato as a culture medium. Schroeder,in 1872, 

 noticed that the growth which occurs on a cooked potato when 

 it is exposed to the air is due to bacteria. He also used 



