HANGING-DROP CULTURES 71 



MISCELLANEOUS METHODS. 



Hanging-drop Cultures. It is often necessary to observe 

 micro-organisms alive, either to watch the method and rate of 

 their multiplication, or to investigate whether or not they are 

 motile. This is effected by making hanging-drop cultures. The 

 method in the form to be described is only suitable for aerobes. 

 For this special slides are necessary. Two forms are in use, and 

 are shown in Fig. 27. In A there is ground out on one surface 

 a hollow having a diameter of about half an inch. That shown 

 in B explains itself. The slide to be used and a cover- glass are 

 sterilised by hot air in a Petri's dish, or simply by being heated 



A 



FIG. 27. 



A. Hollow-ground slide for hanging-drop cultures, shown in plan and section. 

 B. Another form of slide for similar cultures. 



in a Bunsen and laid in a sterile Petri to cool. In the case of 

 A, one or other of two manipulation methods may be employed. 

 (1) If the organism be growing in a liquid culture, a loop of 

 the liquid is placed on the middle of the under surface of the 

 sterile cover-glass, which is held in forceps, the points of which 

 have been sterilised in a Bunsen flame. If the organism be 

 growing in a solid medium, a loopful of sterile bouillon is 

 placed on the cover-glass in the same position, and a very small 

 quantity of the culture (picked up with a platinum needle) is 

 rubbed up in the bouillon. The cover is then carefully lowered 

 over the cell on the slide, the drop not being allowed to touch 

 the wall or the edge of the cell. The edge of the cover-glass is 

 covered with vaseline, and the preparation is then complete and 



