200 PATHOGENIC BACTERIA. 



out by Garini, 1 who found that many of the peptones 

 upon the market were impure, and on this account failed 

 to show the indol reaction for bacteria known to produce 

 indol. He recommends the use of the biuret reaction 

 for testing the peptone to be employed. The reagent 

 used is Fehling's copper solution, with which pure pep- 

 tone strikes a violet color not destroyed upon boiling, 

 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 2 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 is as follows: beef-infusion, prepared either 

 by extracting in the cold or at 6o° 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 



1 Centralbl. f. Bakt. u. Parasitenk., xiii., p. 790. 



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



