162 BACTERIOLOGY. 



medium in which they are growing. When deep 

 down in the medium they are usually round, oval, or 

 lozenge-shaped ; whereas when on the surface of the 

 gelatin or agar they may take quite a different form. 

 This is purely a mechanical effect due to the pressure 

 of, or resistance offered by, the medium surrounding 

 them, and is always to be borne in mind, otherwise 

 errors are apt to arise. 



PURE CULTURES. If from one .of these small col- 

 onies a bit be taken upon the point of a sterilized plati- 

 num 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 to 

 which all organisms must be brought before a system- 

 atic study of their many peculiarities is begun. Some- 

 times several series of plates are necessary before the 

 organisms can be obtained pure, but by patiently fol- 

 lowing this plan the results will ultimately be satis- 

 factory. 



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 sep- 

 arated from other colonies as possible. Sterilize in a 

 gas-flame a straight platinum-wire needle. The glass 

 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 five 

 or ten seconds, take up carefully a portion of the colony. 



