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. 



g. Make plates in gelatin and isolate the organisms. 



Note. — ^The precipitated casein has to be washed in order 

 to remove all traces of millf 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. Inoctdate 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 sulphiiric acid. Follow this up with 

 a few drops of a o . 001 per cent solution of potassium 

 or sodium nitrite. 



The appearance of a red color indicates the presence 

 of indol. 



