66 THE BACTERIOLOGICAL EXAMINATION OF MILK 



warmed by being placed in the incubator at 37° C. for from twenty 

 minutes to half an hour. 



3. Stroke cultivation on agrar— 



1. Prepare an agar plate by pouring the contents of a tube of 

 liquefied nutrient agar into a sterile Petri dish, and allow it to 

 solidify by placing the dish on a cold stone slab or other level 

 surface. 



2. When solidification has taken place and the medium has 

 thoroughly hardened, remove on the extreme point of a platinum 

 needle a minute quantity of the material from which it is desired 

 to isolate the organisms, or, in the case of milk, dip the needle 

 in the liquid. 



3. Raise carefully the cover of the Petri dish and, beginning at 



one side of it, draw with the 

 point of the needle a series of 

 parallel lines across the medium 

 at a distance of \ inch apart, 

 without recharging the needle- 

 At each stroke the needle leaves 

 upon the medium a certain 

 number of organisms, the number 

 Fig. II.— I Eolation by stroke culture. becoming fewer and fewer, and 



the individual organisms more 

 and more scattered, as the lines proceed. 



4. Incubate at 37° C. or any desired temperature. The colonies 

 will be found to develop abundantly on the early tracks of the 

 needle and to become more and more scattered and discrete on the 

 later ones. 



5. Remove from the more isolated colonies a small portion of the 

 growth for microscopic examination and the inoculation of media. 



4. Upon nutrient agrar by Klein's method— 



See p. 88 under Isolation of Anaerobes. 



5. Upon blood serum— 



Apparatus required. — A thick platinum needle (No. 18 or 19, 

 Imperial gauge, will be found a convenient size) flattened at the 

 end in spatula form, and three tubes of solidified blood serum. 



Procedure— 



I. Remove upon the flattened portion of the platinum needle 

 a small quantity of the material or liquid under examination and, 



