42 STUDY AND IDENTIFICATION OF BACTERIA 



of the media. Therefore we have superficial and deep colonies. Ex- 

 cept to the person of great experience, all deep colonies look alike and 

 there is at times great difficulty in deciding whether a colony is deep or 

 superficial. It is in the matter of trying to obtain information from the 

 differences in deep colonies that the greatest difficulties in the study 



FIG. 8. 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.} 



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. 

 The material as pus, faeces, throat membrane, etc., should be evenly 

 distributed in a tube of sterile water or bouillon; the swab which was 



