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. Unless we work with pure cul- 

 tures 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 produced. With regard 

 to the pathogenic organisms, or disease germs, Koch 

 laid down certain conditions which have been termed 

 " Koch's Postulates " (p. 175), which should be complied 

 with before the relation of an organism to a disease 

 process can be said to be completely demonstrated, 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 the 

 vessels, instruments, and culture media must be sterile, 

 that is, free from any living organisms, and the entrance 

 of organisms from without must be prevented and con- 

 tamination avoided. In order to sterilise vessels, instru- 

 ments and culture media, heat is generally employed, 

 though for fluid culture media, such as blood-serum, 



M.B. 4 



