COLONY ISOLATION 



49 



differences in deep colonies that the greatest difficulties in the study 

 of bacteriology arise. By using the method of simply stroking plates 

 along five or six parallel lines from one side of the plate to the other 

 with a bent glass rod, platinum loop, or a small cotton swab, we obtain 

 colonies which are well separated and which are entirely superficial. 



FIG. 9. Petri agar plate. Made by spreading scrapings from the mouth over 

 sterilized nutrient agar; after forty-eight hours in the thermostat the light "colonies" 

 develop. Streaked plate. (Delafield and Prudden.) 



We pour about 10 c.c. of agar, blood agar or Endo media into Petri 

 dishes and keep them in the refrigerator for immediate use. 



Smeared or Stroked Poured Petri Plates. The material as pus, faeces, throat 



membrane, etc., should be evenly distributed in a tube of sterile water or bouillon; 



the swab which was originally used for obtaining the material being then pressed 



against the sides of the test-tube to express excess of fluid and then stroked gently 



4 



