THE STUDY OF MICROORGANISMS IN THE SOIL 135 



Cover the tubes with tinfoil and incubate at 60 to 65°C. 



As soon as the cultures show active fermentation make 1 cubic 

 centimeter transfers to tubes of fresh media. This enrichment 

 process may be repeated several times. 



Examine young and old cultures under the microscope. 



Exercise 57 



Isolation of Cellulose Decomposin-g Bacteria 



Select samples of soil from plots which have received applica- 

 tions of straw or materials rich in cellulose. From these samples 

 prepare dilutions of about 1 : 10,000 and 1 : 100,000. Instead 

 of soil, enrichment cultures may be used (see Ex. 54, 55). If 



Fig. 18. — Bacillus cellulosae dissolvens attacking filter paper. (Khouvine.) 



enrichment cultures are used the dilutions should be greater than 

 those from soil. 



Pour plates of the dilutions from soil or enrichment cultures 

 with the cellulose agar medium (Medium 88). 



Because of the long period of incubation, 2 to 4 weeks, it is 

 desirable to keep the plates in a moist chamber, under bell jar 

 at 28°C. At regular intervals, 5, 10, 20, and 30 days, examine 

 the plates. Note change in color and clear zones throughout 

 the opaque agar. 



Make microscopic mounts — nigrosin and carbol fuchsin. 



