222 BACTERIOLOGY 



The methods employed for its detection are as follows: 

 cultivate the organism for twenty-four to forty-eight hours 

 at a temperature of 37 C., in the simple peptone solution 

 known as "Dunham's solution" (see formula for this 

 medium). This solution is preferred because its pale color 

 does not mask the rose color of the reaction when the 

 amount of indol present is very small. 



Four tubes should always be inoculated and kept under 

 exactly the same conditions for the same length of .time. 



At the end of twenty-four or forty-eight hours the test 

 may be made. Proceed as follows: to a tube containing 

 7 c.c. of the peptone solution, but which has not been inocu- 

 lated, add 10 drops of concentrated sulphuric acid. To 

 another similar tube add 1 c.c. of a 0.01 per cent, solution 

 of sodium nitrite, and afterward 10 drops of concentrated 

 sulphuric acid. Observe the tubes for five to ten minutes. 

 No alteration in their color appears, or at least there is no 

 production of a rose color. They contain no indol. 



Treat in the same way, with the acid alone, two of the 

 tubes which have been inoculated. If no rose color appears 

 after five or ten minutes, add 1 c.c. of the sodium nitrite 

 solution. If now no rose color is produced, the indol reac- 

 tion may be considered as negative i. e., no indol has been 

 formed as a product of the growth of the bacteria. 



If indol is present, and the rose color appears after the 

 addition of the acid alone, it is plain that not only indol 

 has been formed, but coincidently a reducing-body. This is 

 found, by proper means, to be nitrous acid. The sulphuric 

 acid liberates this acid from its salts and permits of its 

 reducing action being brought into play. 



If the rose color appears only after the addition of both 

 the acid and the nitrite solution, then indol has been formed 

 during the growth of the organisms, but no nitrites. 



