THE EXAMINATION OF CULTURES 39 



EXERCISE XII 



THE EXAMINATION OF CULTURES ON SPECIAL MEDIA 



63. As certain of these media are used to determine the 

 effect of the bacteria upon them, it is important to observe very 

 carefully not only the appearance of the gro^yth of the bacteria, 

 but also their effect, if any, upon the medium on or in which 

 they are growing. This is especially noticeable in the milk,, 

 litmus milk, and su gar boui ll on cultures . The changes here' are 

 largely due to the action of the bacteria on the sugars or their 

 power to produce alkali. 



64. Work for this Exercise. — Examine and describe the 

 cultures made on the special media in Exercise XI. 



Examine the bacteria on the potato culture microscopically 

 (i) in the fresh condition (hanging-drop preparation), and 

 (2) in stained cover-glass preparations. Stain a preparation 

 with G^^0l:;fe^siTr°and one with an aqueous solution of gen- 

 tian-violet. Describe the appearance of the bacteria (33) and 

 make a drawing of a few of them. 



65. A Few Points to be observed in Studying Cultures on 

 Special Media : — 



(a) Potato. Note carefully the extent and color of the 

 growth and its consistence. 



((5) Milk. Note whether or not the general appearance and 

 odor of the milk has been changed, observe whether the casein 

 has been coagulated, giving a firm, soUd coagulura, or precipi- 

 tated. Is the coagulura covered with a Uqiiid (serum)=, if so, is 

 it clear or milky? Is there any appearance suggestive of sa- 

 ponification ? Determine its consistence, chemical reaction as 

 indicated by htmus paper (26), and give as descriptive a name 

 as possible to its odor. 



{c) Litmus milk. Note especially whether there has been 



