130 BACTERIOLOGY. 



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 dhTerent 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 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 to which all 

 organisms must be brought before a systematic study of 

 their many peculiarities 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 sep- 

 arated from other colonies as possible. Sterilize in the 

 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. Guard against touching anything but the colony. 



