22 



EXERCISE VI. 



THE EXAMINATION OF CULTURES. 



21. Explanatory note. In studying cultures of bac- 

 teria, it is necessary to observe very carefully (i) the mac- 

 roscopic appearance of the growth in or upon the media, 



(2) the microscopic appearance of the bacteria in (a) the 

 living condition (hanging-drop preparation), and (b) in the 

 dead and stained condition (cover-glass preparation), and 



(3) the effect of the growth of the bacteria upon the chemi- 

 cal and the physical properties of the medium. To de- 

 termine these, the cultures must be kept under observation 

 for several days and often weeks. A careful record should 

 be made of the changes observed in- the appearance of the 

 cultures. Illustrate with drawings. 



22. General directions. Examine carefully and 

 describe fully the appearance of the bouillon, agar, and 

 gelatin cultures made in Exercise V. 



Determine the reaction of the bouillon culture and note 

 whether there is any change in its consistence (viscidity). 



Make a hanging drop preparation from each culture and 

 examine and describe the appearance of the bacteria in each. 



Make a drawing of the gelatin and slant agar cultures and 

 also of a few of the bacteria in one of the hanging-drop prep- 

 arations. 



Read the paragraphs in the text books on the examina- 

 tion of cultures and hanging drop preparations. 



23. Suggestions for the macroscopic examination. 

 The external appearance of cultures should be observed and 

 noted on the day after they are made and on each succeed- 

 ing day until the growth ceases. In bouillon cultures the 

 appearance of the liquid, whether uniformly, faintly or 

 heavily clouded, turbid, clear or clouded with flocculent 

 masses held in suspension, the quantity, and nature of sedi- 



