CHAPTER V. 



The principles involved in the methods of isolation of bacteria in 

 pure culture by the plate method of Koch— Materials employed. 



Since the introduction of the plate method for iso- 

 lating in pure culture the individual species from mix- 

 tures of bacteria, a number of modifications have been 

 adopted, but the principle of them all is the same. 

 The observation which led to their development was 

 a very simple one, and one that is commonly before 

 us. Koch noticed that on solid substances, such, for 

 example, as a slice of potato, which had been exposed 

 for a time to the air and which afforded proper nourish- 

 ment for the lower organisms, there developed after a 

 short time small patches of material which proved to be 

 colonies of bacteria. Each of these colonies on closer ex- 

 amination showed itself to be, as a rule, composed of but 

 a single species. There was no tendency toward a con- 

 fluence of these colonies, and from the differences in 

 their naked-eye appearances, it was easy to see that they 

 were mostly the outgrowth of different species of bacteria. 



The question that then presented itself was : If from 

 a mixture of organisms floating in the air it is possible 

 in this way to obtain in pure cultures the individual 

 organisms composing the mixture, what means can be 

 employed for obtaining the same results at will from 

 mixtures of different organisms when found under other 

 conditions ? 



