32 LABORATORY BACTERIOLOGY 



Count the number of colonies in several (20 to 40) of the 

 areas and multiply the mean number by the number of areas 

 covered. This product gives the fipproximate number of 

 colonies on the plate. 



48. Making Subcultures from Colonies. — Select the tubes 

 of media to be used, and flame the mouths as heretofore de- 

 scribed. 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 medium, and inoculate it. If bouillon is used first, a tube 

 of agar or gelatin can be inoculated immediately afterward, 

 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 rejected, the cover can be entirely 

 removed in the beginning. The newly inoculated tubes or 

 subcultures should be labelled 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 growths 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 single particles of dust often 

 carry two or more bacteria. 



