136 BACTERIOLOGY. 



during manipulation the needle touches anything else 

 whatever than the colony from which the culture 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 result- 

 ing from the inoculation of this bit of colony into a 

 fresh sterile medium will be pure. 



In the meantime have in the other hand a test-tube 

 of sterile medium: gelatin, agar-agar, or potato. This 

 tube is held across the palm of the hand in an almost 

 horizontal position with its mouth pointing out between 

 the thumb and index finger and its contents toward the 

 body of the worker. With the disengaged fingers of the 

 other hand holding the needle the cotton plug is removed 

 from the tube by a twisting motion and placed between 

 the index and second fingers of the hand holding the 

 tube, in such a way that the portion of the plug which 

 fits into the mouth of the test-tube looks toward the 

 dorsal surface of the hand and does not touch any por- 

 tion of the hand ; this is accomplished by placing only 

 the overhanging portion of the plug between the fingers. 

 The needle containing the bit of colony is now to be 

 thrust into the medium in the tube if a stab-culture is 

 desired, or rubbed gently over its surface if a smear- 

 culture is to be made. The needle is then withdrawn, 

 the cotton plug replaced, and the needle sterilized before it 

 is laid down. Neither the needle nor its handle should 

 touch the inner sides of the test-tube if it can be avoided. 



The tube is then labelled and set aside for observa- 

 tion. The growth which appears in the tube after 

 twenty-four to thirty-six hours will be a pure culture 

 of the organisms of which the colony was composed. 



