34 LABORATORY BACTERIOLOGY 



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

 areas and multiply the mean number by the number of areas 

 covered. This product gives the approximate number of 

 colonies on the plate. 



50. Making subcultures from colonies. Select the tubes 

 of media to be used and flame the mouths as heretofore 

 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 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 charge the needle again from the 

 colony. If the plate is to be rejected, the cover may 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 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. Single particles of dust often carry 

 two or more bacteria. 



51. Chester's terminology for description of colonies. 



I. Form of colonies. Plate culture. 



Pjinctiform : dimensions too slight for defining form by 



naked eye, minute, raised, semispherical. 

 Round: of a more or less circular outline. 

 Irregiilar. 

 Elliptical. 



Fusifo?-m : spindle-shaped, tapering at each end. 

 Cochlcatc : spiral or twisted like a snail shell. 



