HANGING-DROP CULTURES 



69 



has not yet been determined whether this is the whole explana- 

 tion of Tarozzi's method. 



When it is desired to grow anaerobes on the surface of a 

 solid medium such as agar, tubes of the form shown in Fig. 24, 

 a and b, may be used. A stroke culture having been made, the 



Fig. 24. — Tubes for anaerobic cultures on the surface of solid media. 



air is replaced by hydrogen as just described, and the tubes are 

 fused at the constrictions. Such a method is of great value 

 when it is required to get the bacteria free from admixture of 

 medium, as in the case of staining flagella. 



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. 

 Tor this special slides are used. Two forms are in use, and 

 are shown in Fig. 25. 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 

 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 ve.ry 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 



