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 isolation 

 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 remains the same. The 

 observation that lead to their development is of almost 

 daily occurrence. When bread, cooked potatoes or old bits 

 of leather are left in moist, damp surroundings they invari- 

 ably become "moldy" as we call it; that is to say, they 

 become more or less covered or spotted with deposits that 

 are known to Jbe composed of living micro-organisms. 



If one watches the evolution of this condition from day 

 to day it will be seen that the moldy deposit begins as a 

 number of small isolated points which, as they get larger, 

 may finally coalesce into a confluent mass that eventually 

 covers the surface. If one examine these points, however, 

 before they begin to run together, it is found that they are 

 composed of micro-organisms of several different kinds, 

 some being molds, some yeasts, and some bacteria. The 

 isolated growths of these various species present different 

 naked-eye appearances, so that even at a glance we are 

 justified in suspecting that they are of a different nature. 

 They develop from single cells that have fallen upon the 



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