108 BACTERIOLOGY 



On the gelatin plates from the original tube, as was expected, 

 the colonies were too numerous to be of use; on the plates 

 made from the first dilution they were much fewer in number, 

 but usually they were still too numerous and too closely 

 packed to permit of characteristic growth; on the second 

 dilution they were, as a rule, fewer in number and widely 

 separated, so that the individuals of each species were in 

 no way prevented by the proximity of their neighbors from 

 growing each in its typical way. (See Fig. 15.) There 



FIG. 15 



Series of plates showing the results of dilution upon the number of 

 colonies: A, Plate No. 1, or "original;" B, first dilution, or Plate No. 2; 

 C, second dilution, or Plate No. 3. About one-fourth natural size. 



was then no difficulty in picking out the colonies resulting 

 from the growth of the different individual bacteria. This, 

 then, is the principle underlying Koch's method for the 

 isolation of bacteria by means of solid media. 



The fundamental constituent of the media employed is 

 the bouillon, which contains all the elements necessary for 

 the nutrition of most bacteria, the gelatin being employed 

 simply for the purpose of rendering the bouillon solid. The 

 medium on which the organisms are growing is, therefore, 

 simply solidified bouillon, or beef -tea. 



