58 GENERAL MICROBIOLOGY 



EXERCISE 15. ISOLATION OF MICROORGANISMS 

 FROM PLATE CULTURES AND METHOD OF 

 MAKING AGAR STREAK CULTURE 



Apparatus. Straight platinum needle; several tubes 

 of sterile agar slants; Bunsen burner; wax pencil; plate 

 containing from 30 to 200 well-separated colonies. 



Method. 1. Examine the plate to determine the colo- 

 nies which differ macroscopically and microscopically, (Use 



FIG. 19. Various Forms of Platinum Needles. (Orig. Northrup.) 



a counting lens or the lowest power of a compound micro- 

 scope.) 



2. Note the most isolated of each kind, and mark them 

 with the wax pencil upon the bottom of the plate to insure 

 picking up the proper colonies later. Also note how the 

 deep and surface colonies differ. 



3. Examine each marked colony under the lowest power 

 of the microscope to make sure of its purity. If the colony 

 does not appear to be wholly isolated, pick up a small 

 portion of it with a sterile platinum needle and stain with 

 one of the common stains (see p. 88) or examine it in the 



