﻿IX, B, 4 Barber: The Pipette Method 329 



4. In some cases it is convenient to transfer a series of 

 isolated organisms, each to a separate test tube, without the 

 loss of time necessary for a change of pipettes at each transfer. 

 For this purpose the large cover is slipped to the left, so that 

 about 2 centimeters of the top of the isolating chamber is free. 

 An oblong piece of mica, about 2.5 centimeters by 4,5 centi- 

 meters in size and provided with a circular perforation about 

 5 millimeters in diameter, is sterilized in the flame and placed 

 over the opening with its free edge in contact with that of the 

 cover. A small sterile cover glass is placed over this opening. 

 The isolated organism is taken up in a pipette and deposited 

 on the smaller cover. The cover is then taken up in sterile for- 

 ceps and placed into a liquid medium or on the surface of a solid 

 medium in a test tube or Petri dish. Or a flattened platinum loop 

 is moistened in a sterile fluid and placed with the flat side in 

 contact with the top of the small cover. The cover will adhere to 

 it, and may be easily lifted and transported. A new small cover 

 is now put over the opening and a second organism placed 

 under it. If necessary, the mica may be sterilized in the flame 

 before receiving another cover. With organisms very sensitive 

 to drying, it is best to place a small drop of broth on the 

 underside of the cover before placing it on the mica. Where 

 such a drop is used, the mica may be placed on the left end of the 

 box, as shown in fig. 9. The isolated organism on the larger 

 cover will, in that case, be better protected against drying, 

 and it will not be necessary to cover the opening on removal of 

 the mica for sterilization. It may be unnecessary to isolate 

 organisms previous to transfer if the drop containing the 

 bacteria is shallow and the bacteria relatively few, so that one 

 can be sure that only one bacterium enters the pipette. Or, if 

 a small drop contains a known number of bacteria, they may 

 be removed and transferred one at a time. In any case, the 

 bacterium may be inspected in a droplet on the small cover 

 to make sure of its isolation. When these small covers are 

 removed to a solid medium, opportunity is aiforded for observing 

 the growth of the bacterium between the cover and the medium. 



5. By attaching two holders to the stage of the microscope, 

 two pipettes may be used simultaneously (fig. 15) . An organism 

 may be isolated with one pipette and immediately picked up by 

 the other and transferred, or may be allowed to grow in the 

 second pipette. By the use of either this method or the one 

 described under 4, one may remove bacteria from a test tube, 

 isolate a single organism, and place the isolated organism into 

 a new test tube after a stay of only a minute or so on the cover 



