54 General Bacteriology. 



b. Place in neck of tube a piece of filter paper which has been dipped in Nessler's 

 reagent (for formula see works on water analysis). A yellow to reddish brown color 

 indicates the presence of ammonia. 



REFERENCES. L. & N. 141. 



SPECIAL DIRECTIONS. Use sewage to inoculate medium. 



EXERCISE LIII. DETECTION OP SULPHURETTED HYDROGEN. 



GENERAL DIRECTIONS. 



a. Make a culture in a test-tube, or better, a flask of bouillon and incubate at 

 38 C. 



b. Twenty-four hours later fasten in the flask, by means of the cotton plug, a strip 

 of filter paper moistened with lead acetate. 



c. The presence of sulphuretted hydrogen is indicated by change of color from 

 brownish to blue. The color change is often slight and can be best detected by frequent 

 observations. 



REFERENCES. L. & N. 138. 



SPECIAL DIRECTIONS. Use B. coli or sewage. 



EXERCISE LIV. DETECTION OF INDOL. 



GENERAL DIRECTIONS. 



a. Make a culture in a tube of glucose-free broth* (or Dunham's solution). 



b, 24 hours to 1 week later add a few drops of concentrated sulphuric acid and 1 cc. 

 of sodium nitrite solution. (Sodium nitrite, 0.02 gms. Distilled water, 100 gms.) 



The presence of iudol is indicated by the production of a deep red color. 

 REFERENCES. L. & N. 142; McF. 56; M. & R. 87. 

 SPECIAL DIRECTIONS. Use B. coli. 



EXERCISE LV. DETERMINATION OF CHEMICAL ENZYHES IN CULTURES. 



GENERAL DIRECTIONS. 



a. Make two gelatin stab cultures of a rapidly liquefying organism and incubate 

 several days or until the gelatin has all been liquefied. 



b. Pour one into a tube of gelatin to which carbolic acid (yV cc. of a 5% sol. per 

 cc. of medium) has previously been added. Mark the line which separates the liquid and 

 solid gelatin. 



c. Add the other tube of liquefied gelatin to a tube of carbolized milk. 



d. Make control cultures in the carbolic media with a pure culture of the organism 

 used above to show that the acid inhibits the growth and that the changes are not due 

 to the living organism. 



REFERENCES. McF. 53. 



SPECIAL DIRECTIONS. Use B. subtilis. 



EXERCISE LVI. VARIATION IN ENZYME PRODUCTION. 



Make stab cultures of Pseudomonas aeruginosa (SCHROETER) MIG. (B. pyocyaneus), 

 or any slow liquefier, in ordinary neutral gelatin and also glucose gelatin. Compare 

 rate of liquefaction in each. 



*This is prepared from beef by inosulating the meat infusion with an organism capable of fer- 

 menting sugar, such as B. coli, and allowing it to stand several hours at !)8 C. The meat is then 

 strained and the bouillon prepared in the usual manner. This is recommended for testing for indol. 



