208 



Cultures, and their Study 



Colonies. The progeny of each bacterium form a mass which is 

 known as a colony. When these are separated from one another, 

 each is spoken of as a single colony, and different characteristics 

 belonging to different micro-organisms enable us at times to recognize 

 by macroscopic and microscopic study of the colony the particular 

 kind of micro-organism from which it has grown. The illustrations 

 show the various types of colonies and the legends the terms used 

 in describing them. 



Growing colonies should be observed from day to day, as it not 

 infrequently happens that unexpected changes, such as pigmenta- 



Fig. 56. Types of colonies: a, Cochleate (B. coli, abnormal form) ; b, conglom- 

 erate (B. zopfii); c, ameboid (B. vulgatus) ; d, filamentous (Frost). 



tion and liquefaction, develop after the colony is several days old 

 and indeed sometimes not until much later. Again, many colonies 

 make their first appearance as minute, sharply circumscribed points, 

 and later spread upon the surface of the culture-medium, either in 

 the form of a thin, homogeneous layer or a filamentous cluster. It 

 is particularly important that in describing new species of bac- 

 teria an account of the appearance of the colonies from day to day, 

 comparing all of their variations for at least two weeks, should 

 be included. 



Tig. 57- Surface elevations of growths: a, Flat; b, raised; c, convex; d, pulvi- 

 nate; e, capitate;/, umbilicate; g, umbonate (Frost). 



Pure Cultures. Single colonies also subserve a second very im- 

 portant purpose, that of enabling us to secure pure cultures of bacteria 

 from a mixture. For this purpose an isolated colony is selected and 

 carefully examined to see that it is single and not a mixture of two 

 closely approximated colonies of different kinds, and then trans- 

 planted to a tube of an appropriate culture-medium. If the colonies 

 are few and of good size, each is picked up with a sterile platinum 

 wire and transplanted to a tube of appropriate culture-medium. 

 If, however, the colonies are numerous, of small size, and close to- 



