32 



EXERCISE IX. 



THE EXAMINATION OF PLATE CULTURES AND MAKING 

 SUBCULTURES FROM COLONIES. 



§ 40. Explanatory note. In practical bacteriological 

 work plate cultures are made use of in determining (i) the 

 number of bacteria there is in a given substance ; (2) the 

 different species of bacteria present, and (3) the character of 

 the growth in a colony of the organism in question. Other 

 important facts, such, for example, as the relative number 

 of each species of bacteria, or the difference in the appear- 

 ance of the surface and deep colonies are learned through 

 this process. The plate culture, therefore, is one of the 

 most important single methods employed in isolating and 

 studying bacteria. 



§41. General directions. Examine carefully and 

 describe the plate cultures made in Exerci.se VIII. If the 

 agar plates do not have colonies, or if the colonies are so 

 numerous that they cannot be counted on any of the plates, 

 make the cultures over again , and give an explanation in the 

 notes of this exercise for the failure to obtain good results. 



Make a hanging drop preparation from a colony from an 

 agar plate, and one from a colony from a gelatin plate, and 

 examine them microscopically. Describe the appearance of 

 the bacteria in each. 



Make a cover-glass preparation from the same colonies 

 and stain each with carbol fuchsin. Examine each carefully 

 and make a drawing of a few of the isolated bacteria. 

 Describe (§30) the appearance of the bacteria in these 

 preparations. 



Inoculate a tube of bouillon, one of agar and one of gela- 

 tin from a well isolated colony from one of the agar plates. 



§ 42. Suggestions for the examination of the plate and 

 roll cultures. Observe the general appearance of the 



