CHAPTEE IV. 



Principles involved In the methods of isolation. of hacteria in pure culture 

 by the plate method of Koch— Materials employed. 



As was stated ia the introductory chapter, the isola- 

 tion in pure cultures of the different species of bacteria 

 from mixtures of these organisms was rendered possible 

 only through the methods suggested by Koch. Since 

 the adoption of these methods they have undergone 

 many modifications, but the principle involved has 

 remained unaltered. The observation which led to 

 their development was a very simple one, and one that 

 is commonly before us. Koch noticed that on solid sub- 

 stances, such, for example, as a slice of potato or of bread, 

 which had been exposed for a time to the air and which 

 afforded proper nourishment 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 examination showed itself 

 to be, as a rule, composed of but a single species. 

 There was no tendency on the part of these colonies to 

 become confluent, 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 ? 



