THE CULTIVATION OF MICRO-ORGANISMS 113 



Colonies are picked out with sterile glass capillaries, and deeper 

 colonies are reached by breaking the tube. The successful use 

 of the method requires some practice. 



Appearance of the Colonies. The colonies obtained in the 

 Petri dishes or roll-tubes (Fig. 43) may be studied with a hand- 

 lens or with a low power microscope. In the latter case, use the 

 plane mirror with the iris diaphragm nearly closed. The colonies 

 present various appearances. Some of them are white, some 

 colored; some are quite transparent and others are opaque; some 

 are round, some are irregular in outline; some have a smooth 

 surface, others appear granular, and others present a radial 

 striation. Surface colonies often present different appearances 

 from those occurring more deeply. Surface colonies are likely 

 to be broad, flat and spreading. If the colony consists of bacteria 

 which have the property of liquefying gelatin, a little funnel- 

 shaped pit or depression forms at the site of the colony. The 

 appearance of colonies may be of great assistance in determining 

 the character of doubtful species. The appearance in gelatin 

 plates of the colonies of the spirillum of Asiatic cholera, for in- 

 stance, is one of the most characteristic manifestations of this 

 organism. 



Pure Cultures. From these colonies pure cultures may be 

 obtained by the process called "fishing." Select a colony from 

 which cultures are to be made; touch it lightly with the tip of a 

 sterilized platinum wire, taking great care not to touch the me- 

 dium at any other point. Introduce the wire into a tube of gelatin 

 after removing the plug and flaming the mouth of the tube. 

 Sterilize the wire and plug the tube. In a similar manner, and 

 from the same colony, inoculate tubes of agar, bouillon, milk, 

 potato and blood-serum. Gelatin tube cultures are usually inocula- 

 ted by introducing the platinum needle into the medium vertically, 

 making a "stab-culture." Inclined surfaces such as those of 

 agar, potato or blood-serum are inoculated by drawing the wire 

 lightly over the surface of the medium, making a "smear-culture" 

 or "streak-culture" (Figs. 44 and 45). Liquid media are inocula- 

 8 



