THE CULTIVATION OF BACTERIA. IOI 



izing, over the surface of another, perhaps over several in 

 succession. 



Appearance of the Colonies. The colonies obtained in 

 the Petri dishes or roll-tubes (Fig. 32) may be studied 

 with a hand-lens or with a low power microscope. In the 

 latter case, use the concave mirror with the iris diaphragm 

 partly 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 instance, is one of the most 

 characteristic manifestations of this organism. 



Pure Cultures. From these colonies pure cultures may 

 be obtained by what is 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 medium at any other point. Introduce the 

 wire into a tube of gelatin. Sterilize the wire and plug the 

 tube. In a similar manner, and from the same colony, in- 

 oculate tubes of agar, bouillon, milk, potato and blood- 

 serum. At the same time it is well to make a smear prepa- 

 ration from the colony and to stain with one of the aniline 

 dyes so as to determine the morphology of the bacteria. 

 The growths which take place in the tubes should contain 

 one and the same kind of bacteria. As seen under the mi- 



