30 LABORATORY BACTERIOLOGY 



EXERCISE IX 



THE EXAMINATION OF PLATE CULTURES AND THE 

 MAKING OF SUBCULTURES FROM COLONIES 



44. In practical bacteriologic 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 pres- 

 ent, 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 appearance of the surface and deep colonies, 

 are learned through this process. The plate culture, there- 

 fore, is one of the most important single methods employed 

 in isolating and studying bacteria. 



45. Work for this Exercise. — Examine carefully and de- 

 scribe the plate cultures made in Exercise 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 each of the same 

 colonies and stain each with carbol fuchsin. Examine each 

 preparation carefully, and make a drawing of a few of the 

 isolated bacteria. Describe (33) the appearance of the 

 bacteria in these preparations. 



Inoculate a tube of bouillon, one of agar, and one of gelatin 

 from a well-isolated colony on one of the agar plates. 



46. Suggestions for the Examination of the Plate and Roll 

 Cultures. — Observe the general appearance of the plates, note 



