230 CIVIC BIOLOGY 



in a street car. (3) Make a culture from a dishcloth that is 

 washed and boiled once a day, and from one that is not. 

 (4) Compare the number of bacteria in rancid and fresh 

 butter. (5) Allow a fly to walk across a plate of sterile 

 gelatin ; record results.' (6) Make a stab culture by running 

 a straight platinum wire, with germs upon it, down through 

 several inches of sterile gelatin in a tube. Upon removing the 

 wire the gelatin closes around the germs left in its track, and 

 serves to cut off the air supply except at the surface. Do you 

 find three classes of bacteria growing in the culture ? 



The excretions of bacteria render the most favorable medium 

 unfavorable. In general, bacteria do not grow as well upon acid 

 as upon slightly alkaline media. (7) Make a culture from the 

 dust of a dark corner of a room ; from a surface in diffused 

 light; from one in bright sunlight. Can you think of more 

 favorable conditions for the growth of bacteria than that 

 offered by the mouth ? How can you keep your teeth from 

 being destroyed by them ? (8) Inoculate a plate with clean- 

 ing of a finger nail, dandruff, single human hair, cat hair. 

 (9) Breathe into a gelatin tube without touching the lips to 

 the glass; make a plate culture. Can the breath carry bac- 

 teria ? (10) Make a plate culture of some of the substance 

 that has gathered upon the back of the teeth. (11) A bacillus 

 has a characteristic growth upon a culture medium. From the 

 appearance of the colonies do your experiments show that you 

 have grown different species of bacteria? Can you see that 

 by selecting a species of bacteria and inoculating a fresh cul- 

 ture with it, and then from it again selecting and inoculat- 

 ing a fresh medium, you would soon obtain a medium with a 

 "pure culture" of that species of bacteria? (12) Can you 

 now explain the need of such rules and precautions as are 

 given in the early part of this chapter ? 



