CHAPTEK 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 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 original 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 organ- 

 isms, 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 little tendency on the part of these colonies to be- 

 come 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 component 

 individuals, what means can be employed for obtain- 

 ing the same results at will from mixtures of different 



