CULTIVATION OF BACTERIA. 137 



while impure peptone gives a red or reddish-yellow pre- 

 cipitate. Both the peptone and copper solution should 

 be in a dilute form to make successful tests. The 

 addition of 4 c.cm. of the following solution 



Rosalie acid, 0.5, 



80 per cent, alcohol, 100. 



makes it become an excellent reagent for the detection 

 of acids and alkalies. The solution is pale rose in color. 

 If the bacterium produces acids, the color fades; if alka- 

 lies, it intensifies. As the color of rosalic acid is destroyed 

 by glucose, it cannot be used in culture-media contain- 

 ing it. 



Theobald Smith calls attention to the fact that Dun- 

 ham's solution is unsuited to the growth of many bac- 

 teria, some failing altogether to grow in it, and recom- 

 mends that, instead, bouillon free of dextrose shall be used. 

 All bacteria grow well in it, and the indol-reaction is 

 pronounced in sixteen-hour-old cultures. His method of 

 preparation 1 is as follows: beef-infusion, prepared either 

 by extracting in the cold or at 60 C., is inoculated in 

 the evening with a rich fluid culture of some acid-pro- 

 ducing bacterium (Bacillus coli), and placed in the ther- 

 mostat. Early next morning the infusion, covered with 

 a thin layer of froth, is boiled, filtered, peptone and salt 

 added and the neutralization and sterilization carried on 

 as usual, 



To test for the presence of indol, the bacterium is 

 planted in the culture-medium, allowed to grow for 

 upward of twelve hours, and then subjected to the com- 

 bined action of a nitrite and chemically pure sulphuric 

 acid. In making the test, Smith adds to each tube i c.cm. 

 of a o.oi per cent, solution of KNO 2 , freshly prepared, 

 and 10 drops of chemically pure H 2 SO 4 . The presence 

 of indol is characterized by the production of a red 

 color. 



1 Journal of Exp. Medicine, vi. } Sept. 5, 1897, p. 546. 



