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 possi- 

 ble only through the methods suggested by Koch. Since 

 the adoption of these methods they have undergone 

 many modifications, but the principle originally 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 

 substances, 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 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 component 

 individuals, what means can be employed to obtain the 

 same results at will from mixtures of different species of 



