86 METHODS USED IN THE STUDY OF BACTERIA 



study of it can proceed with certainty. In the early days of 

 bacteriology, mixed cultures were frequently confused with 

 pure cultures, and curious and very erroneous ideas were 

 obtained which it has taken many years to correct. 



Methods of Isolation 



Fractional Methods. Pasteur and Cohn, working on the 

 problem of artificial cultivation, worked on the supposition 

 that various species of microorganisms, occurring in a mix- 

 ture, will multiply unequally in any given culture medium ac- 

 cording to their different natures. According to this method, 

 one species will attain the ascendency and can be removed 

 and obtained in pure culture. While this idea is partially 

 true, it is almost always the case that the other kinds of micro- 

 organisms in the mixture are not dead but simply inactive, 

 and are likely to be transferred with the others and under 

 different conditions may gain the ascendency. Klebs used 

 much the same method, transferring the most luxuriant 

 growths from culture to culture until the pure culture was 

 obtained. These methods are used only to a very limited 

 extent at the present time, and if we had to depend upon 

 them, our knowledge of bacteria would be limited and our 

 idea of their life history erroneous. 



Physiological Methods. The physiological differences in 

 bacteria have been made use of in a number of special methods. 

 Roberts, in 1874, and Cohn, in 1876, isolated certain spore- 



