CHAPTER IV. 



Principles involved in the methods of isolation of bacteria in pure 

 culture by the plate method of Koch Materials employed. 



As was stated in the introductory chapter, the isola- 

 tion in pure cultures of the different species that may 

 be present in mixtures of bacteria was rendered possible 

 only through the methods suggested by Koch. Since 

 the adoption of these methods they have undergone 

 many modifications, but the fundamental principle re- 

 mains the same. The observation which led to their 

 development was a very simple one, and one that is 

 frequently before us. Koch noticed that on solid sub- 

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

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

 which afforded proper nourishment for the lower organ- 

 isms, there developed after a short time small patches 

 which proved to be colonies of bacteria. Each of 

 these colonies on closer examination proved to be, as 

 a rule, composed of distinct species of micro-organisms. 

 There was little 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 rep- 

 resented, in the main, the development 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 component in- 

 dividuals, what means can be employed for obtaining the 

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