2IO 



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 a ad 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. 49. — 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. 



Fig. so. — Surface elevations of growths : a. Flat; J, raised; c, convex; i, pulvinate; 

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



Pure Cultures. — Single colonies also subserve a second very 

 important purpose, that of enabling us to secui'e pure cultures of bac- 

 teria 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 

 transplanted 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 together, it may be necessary to do it under a dissecting 



