METHODS USED IN CULTIVATION OF BACTERIA 143 



this particular species entirely apart from others, in what is known as 

 "pure culture." The earliest methods for accomplishing this were the 

 methods of Pasteur and of Cohn who depended upon the power of one 

 species to outgrow all others, if cultivated for a sufficient length of time 

 in fluid media. This method, of course, was inadequate in that it was 

 often purely a matter of chance which one of the mixture of species 

 was finally obtained by itself. A later method, by Klebs, depends 

 upon serial dilution, in test tubes of fluid media, by which the eventual 

 transference of one germ only, to the last tube was attempted. Such 

 methods, none of them of great practical value, have been entirely dis- 



FIG. 28. TAKING PLUGS FROM TUBES BEFORE INOCULATION. 



placed by those made possible by Koch's introduction of solid media 

 which may be rendered fluid by heat. 



The methods now employed for the isolation of bacteria depend upon 

 the inoculation of gelatin or agar, when in the melted state, the thorough 

 distribution of the bacteria in these liquids by mixing, and the sub- 

 sequent congealing of these media in thin layers. By this means the in- 

 dividual bacteria, distributed in the medium when liquid, are held apart 

 and separate when the medium becomes stiff. The masses of growth 

 or "colonies" which develop from these single isolated microorganisms 

 are discrete and are descendants of a single organism, and can be trans- 

 ferred, by means of a process known as " colony-fishing," to fresh sterile 



culture media. 

 11 



