202 Cultivation of Micro-organisms 



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* has called attention to the fact that many 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. Selterf finds that the 

 method of Smith gives inferior results to a simple culture-mediunj 

 consisting 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. 



* "Journal of Exp. Medicine," Sept. s, 1897, vi, p. 546. 

 t "Centralbl. f. Bakt. u. Parasitenk.," Orig. li, p. 465. 



