Bacteria, Helpful and Otherwise 443 



them 2 and 3. Leave the dishes in a warm, dark place for two 

 or three days and then examine. Record the number of colonies 

 in each dish. Notice whether molds have formed. State your 

 conclusions. 



To Determine the Presence of Bacteria on the Teeth. 

 Using three prepared Petri dishes, set one aside, without uncov- 

 ering, for a check or control. Label it 1. Uncovering the others, 

 spread on the medium of one of them scrapings from the teeth 

 before brushing, and on the other scrapings from the teeth after 

 brushing. Label these dishes 2 and 3. Leave them in a warm, 

 dark place for two or three days and examine as before. Record 

 the results, stating the number of colonies observed in each case. 

 State your conclusions. 



Experiment to Show the Conditions Favorable to the 

 Growth of Bacteria. Prepare four cultures in Petri dishes. 

 Expose one culture in a warm, dark place having a moist atmos- 

 phere ; a second in a very cold, dark place having a moist atmos- 

 phere ; a third in a warm, light place having a moist atmosphere ; 

 and a fourth in a cold, light place having a moist atmosphere. 

 Leave the cultures for several days and then examine. Count 

 the colonies of bacteria you see in each culture and decide what 

 conditions are most favorable for growth. 



Soil Bacteria. Certain kinds of bacteria, known as soil bac- 

 teria, have the power to decompose, or break up into simpler 

 forms, organic compounds containing nitrogen, and to recom- 

 bine them into nitrates that may be taken up in solution by the 

 root hairs of plants and used as raw material in making plant 

 food. 



These bacteria act on and decompose such organic matter 

 as dead animals and plants, manure and sewage. Were it not 

 for the work of soil bacteria the earth would not be habitable. 

 They act as middlemen between the lifeless and the living 

 world and thus make possible the use of the same elements over 

 and over again in the production of new organic matter. 



Nitrogen-Fixing Bacteria. Although the air we breathe 

 is about 80 per cent nitrogen, the only living creatures that 

 are able to take this nitrogen directly from the air are certain 



