140 GENERAL MICROBIOLOGY 



REFERENCES 



MARSHALL: Microbiology, pp. 153-161, 306-313, 363-365. 

 CONN, H. W.: Bacteria, Yeasts and Molds, pp. 191-193. 

 BESSON, A.: Practical Bacteriology, Microbiology and Serum Therapy, 

 pp. 35-36. 



EXERCISE 47. TO COMPARE MORPHOLOGICALLY 

 PROTOZOA WITH BACTERIA 



Apparatus. Deep culture dish; concave slide; clean 

 cover-glasses; cover-glass forceps; platinum loop; tube 

 of sterile broth; tube of sterile Chinese ink. 



Cultures. Rich soil or slimy leaves from a pond. 



Method. 1. Place the soil or leaves in the deep culture 

 dish. 



2. Fill the dish two-thirds full with tap water and 

 add the contents of a tube of broth. 



3. Keep the dish at room temperature for twenty- 

 four to forty-eight hours. 



4. At the end of the incubation period, make a hanging 

 drop from the supernatant liquid. Before inverting the 

 drop on the slide, add to it that amount of Chinese ink 

 that adheres to the end of a platinum needle. 



By the use of this ink, organisms are brought out by 

 contrast, showing white on a dark field. The organisms 

 are not killed or injured by the ink. 



5. Observe and measure any protozoa, using the lowest 

 power objective with the step micrometer. Record the size 

 in micra. 



6. Roughly sketch the different species observed, giving 

 comparative measurements. 



7. Using the highest power dry objective, observe bac- 

 teria, noting morphology and size. 



8. Draw lines to represent the ratio between the size 

 of the predominant types of each. 



9. Are the protozoa present visible to the naked eye? 



