CHAPTER II 



METHODS OF CULTIVATING AND ISOLATING 

 ORGANISMS 



IT is necessary for the satisfactory study of micro-organisms 

 in their relation to the various processes of infection and 

 disease, of fermentation, putrefaction, and the like, to 

 separate and isolate the different species occurring in a 

 mixture, and, having done so, to cultivate, grow, or 

 propagate each species on suitable soils through successive 

 generations. A slight consideration will show that unless 

 we work with pure cultures that is, cultures consisting 

 of a single species we can never be sure that a particular 

 result is due to a given organism ; in a mixture several or 

 all of the forms present may conduce to the effect pro- 

 duced. With regard to the pathogenic organisms, or 

 disease germs, Koch laid down certain conditions which 

 have been termed " Koch's Postulates " (p. 147), which 

 must be complied with before the relation of an organism 

 to a disease process can be said to be completely demon- 

 strated, one of which is that " the organism must be 

 isolated and cultivated outside the animal body on suitable 

 media for successive generations." 



In order to isolate organisms in a state of purity it is 

 absolutely necessary to employ vessels, instruments, and 

 culture media which are sterile, that is, free from any 

 living organisms, and to possess the means of manipu- 

 lating them in such a way that the entrance of organisms 

 from without is prevented and contamination avoided. 



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