306 APPLICATION OF METHODS OF BACTERIOLOGY 



gested to Koch a means of separating and isolating in pure 

 cultures the component individuals from mixtures of bacteria, 

 and from it the methods of cultivation on solid media were 

 evolved. 



If, without molesting these objects, we continue the 

 observations from day to day, we shall notice changes in the 

 colonies, due to the growth and multiplication of the indi- 

 viduals composing them. In some cases the colonies will 

 always retain their sharply cut, round, or oval outline, and 

 will increase but little in size beyond that reached after 

 forty-eight to seventy-two hours; whereas others will 

 spread rapidly and quickly overrun the surface upon which 

 they are growing, and, indeed, grow over the smaller, less 

 rapidly developing colonies. In a number of instances, 

 if the observation be continued long enough, many of these 

 rapidly growing colonies will, after a time, lose their lustrous 

 and smooth or regular surface and will show here and there 

 elevations, which will continue to appear until the whole 

 surface becomes conspicuously wrinkled. Again, bubbles 

 may be seen scattered through the colonies. These are due 

 to the escape of gas resulting from fermentation, which the 

 organisms bring about in the medium upon which they are 

 growing. Sometimes peculiar odors due to the same cause 

 will be noticed. 



Note carefully all these changes and appearances, as they 

 must be employed subsequently in identifying the individual 

 organisms from which each colony on the medium has 

 developed. 



If we now examine these colonies upon the bread or potato 

 with a hand-lens of low magnifying power, we will be 

 enabled to detect differences not noticeable to the naked 

 eye. In a few cases we may still see nothing more than a 



