146 GENERAL MICROBIOLOGY 



sterilize in the remaining two funnels. Wrap all in paper 

 and sterilize in the hot-air oven. 



3. Shake the broth culture of B. coli and plate, using 

 dilutions 1 : 1,000 and 1 : 10,000. 



4. Filter each dilution (1 : 1,000 and 1 : 10,000) through 

 each of the different substances, catching the filtrate in 

 sterile test tubes. 



5. Plate 1 c.c. from each filtrate immediately and incu- 

 bate the plates at 21 C. 



6. At the end of five days, count the plates. 



7. Which method of filtration is most efficient? Why? 

 What factors could greatly influence the numbers of micro- 

 organisms developing on the plates after filtration? 



8. What methods are most efficient in removing micro- 

 organisms from liquids? Why? 



9. Suggest some natural methods of filtering micro- 

 organisms. 



10. Give in detail the results obtained, state any con- 

 clusions that may be drawn and point out any practical 

 applications. 



REFERENCES 



EYRE: Bacteriological Technic, 2d Ed., pp. 42-48. 

 MARSHALL: Microbiology, pp. 64-67. 



EXERCISE 61. TO DEMONSTRATE PRESENCE OF MI- 

 CROORGANISMS IN AIR, ON DESK, FLOOR, ETC. 



Apparatus. Six sterile Petri dishes; six tubes of sterile 

 agar. 



1. Air. Method. 1. Pour six plates with uninoculated 

 sterile agar and set on a level surface until solid. 



2. Expose one plate for one minute to (a) laboratory 

 air; (6) air of campus; (c) air of your room while sweep- 

 ing or dusting. 



II. Floor. 1. Bend the straight platinum needle tilf 

 it forms a right angle. 



