INTESTINAL GROUP 83 



The fact that this gas remaining in the closed arm is 

 hydrogen may now be proved by tiUing it into the open 

 arm, which has previously been filled with water and 

 closed by the thumb. By holding a burning match 

 over it and quickly releasing the thumb, a slight explo- 

 sion takes place from the sudden combination of the 

 hydrogen with the oxygen of the atmosphere. 



Gas-formation by bacteria does not necessarily 

 depend on the presence of carbohydrates. Nitrogen is 

 often produced from nitrites, and hydrogen sulphid and 

 ammonia from proteids, especially during the process of 

 putrefaction. 



3. Special study B. — Test for indol and nitrites. 

 What is indol ? What biological activity of the organism 

 does the presence of indol or nitrites, or both, indicate ? 



a) Test for nitrites : Add to a culture in Dunham's 

 solution, or, better, in sugar-free broth, successively 1 

 drop of each of the following solutions: 



(i) Sulphanilic acid 0.5 g. 



Acetic acid {2%%) 150 c.c. 



(2) Naphthylaraine chlorid o. i g. 



Distilled water 20 c.c. 



Acetic acid (25^ 150 c.c. 



A yellowish-red or rose-color shows the presence of 

 nitrites. 



h) Test for nitrites and indol combined. 



(i) Add to a culture in Dunham's solution, or sugar- 

 free broth, I or 2 drops pure sulphuric acid. 



(2) Heat gently. Rose-color shows the presence of 

 nitrites and indol. If no reaction takes place, add — 



(3) A few drops of a solution of o.i g. potassium or 

 sodium nitrite in 1000 c.c. water. Rose-color then 

 indicates the presence of indol only. 



