﻿320 The Philippine Journal of Science im 



If the tip is already perforated, one has only to fill the pipette 

 with a small quantity of broth, blow out a droplet, and, with 

 this as a guide, find the tip with the high power. The advantage 

 of the closed point is that it allows one to gauge the size of 

 the opening to suit the organism to be isolated. The breaking 

 off of the point can be more accurately done under the high 

 power, and one can easily find the unbroken point by immersing 

 it in the broth drop near its edge, and with the edge as a guide 

 locate it under the high power. For one unaccustomed to the 

 technique, however, the first-described method of breaking with 

 the low power and making a broth droplet may be found easier ; 

 the tip can usually be safely broken off under the low power. 



For ordinary isolation of bacteria, considerable variation in 

 the size of the opening is allowable — about 2 to 5 microns will 

 be found suitable for Bacterium coli, for example. If too large, 

 say over 15 microns, the difficulty of isolation will be much 

 greater. If too small, it will be found difficult to blow out the 

 broth or to introduce larger bacteria. If the tip has a sealed 

 blunt point, it is sometimes very difficult to break it open ; it is 

 usually best to make a new one at once. 



10. After the tip is located at the center of the high-power 

 field, lower it safely below the level of any hanging drop and 

 bring the hanging drop containing the bacteria into the center 

 of the field, preferably at its edge. If a particular bacterium is 

 to be isolated, bring it to the center of the field and cautiously 

 raise the tip of the pipette until it is just below the surface of 

 the liquid. The tip may be seen as a shadow immediately below 

 the drop. Now, moving the finger along the up-and-down ad- 

 justment of the holder, bring the tip into the drop near the 

 bacterium, then lower it instantly. The bacterium usually enters 

 the pipette by capillarity. Then move the hanging drop out 

 of the field and raise the pipette into contact with a sterile part of 

 the cover glass covered with fine droplets of condensed moisture. 

 Blow out a very small drop. If the bacterium does not appear, 

 move the stage slightly and blow out a second or, if necessary, 

 a third or fourth drop. The droplets should be very small, and 

 should be made near the edge of a hanging drop on one of the 

 lines on the cover glass so that they may be easily located. If 

 the bacterium sought comes out with one or more bacteria, move 

 a field or so away, discharge liquid from the pipette until it 

 comes out free from bacteria, return to the droplet, and if neces- 

 sary dilute with a small quantity of broth. By repeating the 

 selection from the diluted droplet, one can usually isolate the 

 bacterium at once. 



