METHOD OF MAKING STAB CULTURES 61 



EXERCISE 17. METHOD OF MAKING STAB CULTURES 



Apparatus. Tubes of sterile agar or gelatin; straight 

 platinum. needle; Bunsen burner. 



Culture. Pure culture. 



Method. 1. Liquefy the gelatin or agar tube and re- 

 solidify it in a vertical position in cold running water or 

 in some cold place. 



2. With a sterilized straight platinum needle pick up a 

 very little of the culture or colony. 



3. Insert the needle at the middle of the circle made by 

 the surface of the medium and push the needle about 

 5 cms. into the solid medium (within 1 cm. of the bottom 

 of the tube), then withdraw carefully so that the path of 

 the needle be as limited as possible. The microorganisms 

 grow along the path of the needle. 



Avoid having the shoulder of the rod come in contact 

 with the surface of the medium lest its heat disfigure the 

 surface or even kill the microorganisms. The surface of 

 the medium should remain intact during this process. 



4. Replace the plug in the new culture and sterilize 

 the needle. 



EXERCISE 13. PREPARATION OF A GIANT COLONY 



Purpose. To show the development of a single colony 

 of a microorganism. 



Apparatus. Sterile Roux culture flask* or Petri dish; 

 tubes of agar or gelatin. 



Culture. Organism to be studied. 



Method. 1. Melt two tubes of dextrose agar or gelatin. 



Pour into the culture flask. 



Note. Allow the medium to touch and cover one large side only. 



2. Heat in this horizontal position in flowing steam 

 fifteen minutes. 



3. Distribute the medium evenly over the large side, 

 * Most valuable for molds, especially Rhizopus nigricans. 



