34 



covered. This product gives the approximate number of 

 colonies. 



44. Making subcultures from colonies. Select the 

 tubes of media to be used, and flame the mouths as hereto- 

 fore described. Select a colony as well isolated from all 

 others as possible. With the left hand carefully raise the 

 edge of one side of the cover of the Petri dish, and while 

 holding it, touch the colony with the needle, replace the 

 cover, take up the tube of media and inoculate it. If 

 bouillon is used first a tube of agar or gelatin can be inocu- 

 lated immediately afterwards without recharging the needle. 

 If more cultures are to be made it is necessary to again 

 charge the needle from the colony. If the plate is to be re- 

 jected the cover can be entirely removed in the beginning. 

 The newly inoculated tubes or subcultures should be labeled 

 and treated according to the directions heretofore given for 

 handling cultures. These inoculated tubes should be pure 

 cultures. It sometimes happens, however, that what appears 

 to be a single colony consists of the growth of two organisms. 

 If these should be of different species the cultures made 

 from the colony would probably be impure. These impure 

 growths (apparently single colonies) frequently develop on 

 plate cultures exposed to the air for some time. The 

 particles of dust often carry two or more bacteria. 



EXERCISE X. 



THE PREPARATION OF CERTAIN SPECIAL MEDIA. 



45. Explanatory note. In studying the properties of 

 bacteria it is desirable to cultivate them on a number of dif- 

 ferent media. Bouillon, agar, and gelatin are most com- 

 monly used, but others are necessary in determining the 

 cultural peculiarities and important biochemic properties of 

 the organism in question. The cultivation of bacteria upon 



