3i LABORATORY BACTERIOLOGY. 



EXEECISE XXIV. 



TLIE ISOLATION OF NITRIFYING BACTERIA BY THE MAGNESIA- 

 GYPSUM BLOCK METHOD. 



1. Measure off four 100 cc. portions of "solution for ni- 

 trite formation" in 350 cc. Erlenmeyer flasks. 



2. Plug and sterilize as usual. 



3. Inoculate with 10 cc. (= 5 gm. soil) of a 5-minute in- 

 fusion of a fresh sample of soil. 



4. Incubate 7-8 days at 20° C. 



5. Test qualitatively for the production of nitrites by the 

 Griess colorimetric method. 



6. If nitrites are present, by means of a platinum loop 

 inoculate from these cultures on the surface of the magnesia- 

 gypsum blocks, spreading the inoculum uniformly over the 

 surface. 



7. Incubate 10-14 days at 20° C; at the end of six days 

 replacing the liquid surrounding the gypsum block with a 

 I'l c'sh supply. 



8. Inoculate from some well isolated colonies to other 

 j;;\'psum blocks ; incubate as usual and describe carefully the 

 cliaracter of the growth. 



9. Test qualitatively the solution surrounding the blocks 

 for ammonia, nitrites and nitrates. 



10. Examine the pure cultures obtained in hanging-drop 

 and stained preparations and describe the character of the 

 organisms. 



