200 Cultivation of Micro-organisms 



It was for a long time used for the detection of indol. Garini* 

 found that many of the peptones upon the market were impure, and 

 on this account failed to show the indol reaction in cultures of bac- 

 teria known to produce it. He recommends testing the peptone to 

 be employed by the use of the biuret reaction. The reagent em- 

 ployed is Fehling's copper solution, with which pure peptone strikes 

 a violet color not destroyed upon boiling, while impure peptone gives 

 a red or reddish-yellow precipitate. Both the peptone and copper 

 solutions should be in a dilute form to make successful tests. 



The addition of 4 cc. of the following solution — ■ 



Rosolic acid o • S 



Eighty per cent, alcohol 100. o 



makes the peptone solution a reagent for the detection of acids and 

 alkalies. The solution is of a pale rose color. If the organisms cul- 

 tivated produce acids, the color fades; if alkalies, it intensifies. As 

 the color of rosolic acid is destroyed by glucose, it cannot be used in 

 culture-media containing it. 



Theobald Smith f has called attention to the fact that niany bac- 

 teria fail to grow in Dunham's solution, and recommends that, for 

 the detection of indol, bouillon free of dextrose be used instead. All 

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

 teen-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- 

 producing bacterium (Bacillus coli) and placed in the thermostat. 

 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. 



This method is subject to error caused by the presence in the me- 

 dium of indol produced by the colon bacillus. This can be demon- 

 strated if the tests for indol be sensitive. SelterJ finds that the 

 method of Smith gives inferior results to a simple culture-medium con- 

 sisting of water, 90 parts; Witte's peptone, 10 parts; sodium phos- 

 phate, 0.5 part, and magnesium sulphate, o.i part. 



Other culture-media employed for special purposes will be men- 

 tioned as occasion arises. 



* " Centralbl. f . Bakt. u. Parasitenk.," xin, p. 790. 



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



j "Centralbl. f. Bakt. u. Parasitenk.," Orig. Li, p. 465. 



