FERMENTATION METHODS 1 29 



fested by the presence of bubbles. Acids are detected in cultures 

 by the employment of various indicators in the culture media. 

 Litmus, lacmoid, andrade, and neutral red are used for this pur- 

 pose. By titrating bouillon of previous known acidity with a 

 decinormal soda solution, the amount of acid produced by differ- 

 ent bacteria can be estimated. 



Various sugars are fermented by bacteria, and lactic, acetic, 

 and butyric acids are produced. Indol is also produced by many 

 bacteria (colon bacillus, cholera bacillus), and its presence in cul- 

 ture is an important means of identifying different bacteria. The 

 organism to be studied must be grown in culture media known to 

 be free from indol. For this purpose, all meat extracts must be 

 excluded and a simple solution of peptone and salt, run into tubes 

 and sterilized, is used. After bacteria have grown in this media 

 for several days the indol produced, if it is produced, is detected 

 by adding a few drops of pure sulphuric acid. If a red color 

 (nitroso-indol) is not produced, a few drops of sodium nitrite solu- 

 tion (.02 gram to 100 c.c. of water) must be added, and if a pink 

 to deep red color does appear it may be safely assumed that indol 

 is present. 



Ammonia is detected in culture by suspending a piece of paper 

 wet with Nessler's reagent above a bouillon culture of a given or- 

 ganism. If a yellow to brown color is produced ammonia is 

 present. 



Nitrites are detected by growing the organism in a solution of a 

 nitrate (.02 gram potassium nitrate, 10 grams peptone in 1,000 

 c.c. of water). 



Incubate for a week and then add i c.c. each of the following 

 solutions : 



. 



(a) Sulphuric acid .5 gram. 



Acetic acid 150 c.c. 



(6) Amido naphthaline . i gram. 



Water . 20 c.c. 



Boil, filter, and add 180 c.c. of dilute acetic acid. 

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