102 LABORATORY MANUAL OF MICROBIOLOGY 



In order to prevent evaporation all samples must be kept 

 in a moist chamber. A large glass beaker or metal container may 

 be used. Avoid glass bell jars, unless of Pyrex glass, since the 

 high temperature may cause them to crack. 



After 1 week in the incubator, prepare agar plates from the 

 different samples. Pour from dilutions 1:1,000 and 1:10,000. 



The plates must be incubated at 60°C. 



Determine the number of thermophilic bacteria in 1 gram of 

 soil. 



If desirable, a study may be carried on of the bacteria growing 

 at low temperatures. 



Exercise 10 

 Effect of Season of Year on Number of Bacteria 



Collect samples of soil from an alfalfa field, clover field, blue- 

 grass field, orchard sod, etc. If the soil is frozen, it will be 

 necessary to use a pick or hatchet in securing samples. 



Prepare dilution plates as soon as possible after the samples 

 reach the laboratory. Unless the soil is very rich use dilutions 

 of 1 : 10,000 and 1 : 100,000, about four or five parallel plates for 

 each dilution. 



At the same time plates are poured, make moisture determina- 

 tions of the soil. 



Record outside temperature and also soil temperature at the 

 time samples are drawn. Read the directions given in Exercise 6. 



Exercise 11 

 Effect of Plant Roots on Number of Microorganisms 



Collect soil samples from the immediate vicinity of the roots 

 of various plants (alfalfa, clover, etc.), and similar samples 1 or 

 2 feet away from the plants. 



Determine the number of bacteria by means of plate counts. 



Exercise 12 



Effect of Depth of Soil on Number of Microorganisms 



The samples for this exercise should be drawn from virgin soil 

 well removed from any source of contamination. The type of 

 soil will determine to a certain degree the number of organisms at 

 different depths. 



