154 BACTERIOLOGY 



different form. This is purely a mechanical effect due to 

 the pressure of, or resistance offered by, the medium sur- 

 rounding them, and is always to be borne in mind, other- 

 wise false interpretations may be made. 



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 

 organisms 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 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 rnay 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. If during manipulation the needle touches 

 anything else whatever than the colony from which the cul- 

 ture is to be made, it must be sterilized again. This holds 

 not only for the time before touching the colony, but also 

 during its passage into the test-tube from the colony; 

 otherwise there is no guarantee that the growth resulting 



