170 LABORATORY GUIDE IN BACTERIOLOGY 



3. Wash the precipitate with dilute acetic acid until 

 the milk sugar has been removed. 



4. Place in a 500 c.c. flask and add 300 c.c. distilled 

 water. 



5. Neutralize with NaOH and sterilize in the auto- 

 clave. 



6. Inoculate with soil suspension. 



7. Incubate at 37 C. until all the casein has been 

 dissolved. 



8. Determine the composition of the solution as in 

 Exercise i. 



9. Make plates in gelatin and isolate the organisms. 

 NOTE. The precipitated casein has to be washed in order 



to remove all traces of milk sugar, otherwise acid-forming bacteria 

 will multiply rapidly and crowd liquefiers out. 



SECTION 3 



THE FORMATION OF AMIDO COMPOUNDS AND 

 AMMONIA 



EXERCISE I. THE FORMATION OF INDOL 



1. Prepare 200 c.c. Dunham's solution and place 

 in a 500 c.c. flask. 



2. Sterilize in the autoclave. 



3. Inoculate with a pinch of garden earth and incu- 

 bate at 37 C. for one week. 



4. Remove a small amount with a sterile pipette and 

 add a few drops sulphuric acid. Follow this up with 

 a few drops of a o.ooi per cent solution of potassium 

 or sodium nitrite. 



The appearance of a red color indicates the presence 

 of indol. 



