STAB AND SMEAR CULTURES. 101 



the colonies depending upon their location in the medium 

 can usually be detected. When deep down in the medium, 

 owing to surrounding pressure, they are quite round, 

 oval, or lozenge-shape ; whereas, when they are on the 

 surface of the gelatin or agar, they may take quite a 

 different form. This is purely a mechanical effect, and 

 is always to be borne in mind, otherwise errors are 

 apt to arise. 



PURE CULTURES. If from one of these small colonies 

 a bit be taken upon the point of a sterilized platinum 

 needle and introduced into a tube of sterilized gelatin 

 or agar-agar, the growth that results will be what is 

 known as a " pure culture," the condition in which all 

 organisms must be before a systematic study of their 

 many pecularities is begun. Sometimes several series of 

 plates are necessary before the organism can be obtained 

 pure, but by patiently following this plan the results 

 will ultimately be satisfactory. 



TEST-TUBE CULTURES; STAB CULTURES; SMEAR 

 CULTURES. After separating the organisms, the one 

 from the other by the plate method just described, they 

 must be isolated from the plates as pure stab or smear 

 cultures. 



This is done in the following way : Decide upon the 

 colony from which the pure culture is to be made. Select 

 preferably a small colony and one as widely separated 

 from other colonies as possible. Sterilize in the gas- 

 flame a straight platinum-w r ire needle. The glas's handle 

 of the needle should be drawn through the flame as well 

 as the needle itself, otherwise contamination from this 

 source may occur. When it is cool, which is in three 

 to five seconds, take up carefully a portion of the colony. 

 Guard against touching anything but the colony. If, 



