34 LABORATORY BACTERIOLOGY 
Count the number of colonies in several (10 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. 
1. Form of colonies. Plate culture. 
Punctiform: dimensions too slight for defining form by 
naked eye, minute, raised, semispherical. 
Round: of a more or less circular outline. 
Irregular. 
Elliptical. 
Fusiform: spindle-shaped, tapering at each end. 
Cochleate: spiral or twisted like a snail shell. 
