16 A MANUAL OF BACTERIOLOGY 



SECTION IV 



RELATION OF BACTERIA TO FACTORS OF THE 

 PHYSICAL ENVIRONMENT 



Exercise 15. Relation of Bacteria to Oxygen (Growth) 



1. Pour a tube of melted agar into a sterile Petri dish. Keep 

 the dish level until the agar has hardened. 



2. Inoculate the plate in three parallel lines about 5 mm. 

 distant from each other. Use the straight platinum wire charged 



\vith B. subtilis. 



3. Sterilize a plate of mica 

 or a cover glass in the Bunsen 

 flame, wait a few seconds for 

 it to cool, then lay it over the 

 lines of inoculation. Press it 

 FIG. 12. A Petri dish down in firm contact with 



the agar to exclude the air. 



4. After three days examine the plate for bacterial growth. 

 Sketch the plate, showing the location of bacterial growth. 



5. Make a duplicate plate, using an anaerobic germ. 



Exercise 16. Relation of Bacteria to Oxygen (Motility) 



Examine water in which seeds have been boiled and allowed 

 to putrefy for motile bacteria. Transfer a drop containing bac- 

 teria to a slide, add a few filaments of green alga, and seal 

 the edge of the cover glass with vaseline. Clamp the slide to the 

 microscope stage and set the preparation in the dark until the 

 bacteria have ceased to move. Why do they cease to move ? 

 Then set the preparation near a window and look for the resto- 

 ration of movement. Where does it first take place ? What 

 function does the alga filament play ? Why ? 



Exercise 17. Effect of Light upon Bacteria 



1. Inoculate a tube of melted agar with B. prodigiosus. Mix 

 well and transfer three loopfuls to a second melted tube. Pour 

 this second tube into a sterile Petri dish. 



