SPECIAL MEDIA. 121 



the power of bacteria to cause acid fermentation in a 

 non-proteid medium containing a fermentable sugar; 

 and for observing the variations of this power in closely 

 allied though not identical species. 



Dunham's pefptone solution. The medium usually 

 known as Dunham's solution is prepared according to 

 the following formula : 



Dried peptone ... ... 1 part. 



Sodium chloride . . . . . 0.5 " 



Distilled water . 100 parts. 



It is usually of a neutral or slightly alkaline reac- 

 tion, and neutralization is not, therefore, necessary. 

 It is filtered, decanted into tubes or flasks, and ster- 

 ilized in the steam sterilizer in the ordinary way. 

 The most common use to which this solution is put 

 is in determining if the organism under considera- 

 tion possesses the property of producing indol as one 

 of its metabolic products. It is essential for accu- 

 racy that the preparation of dried peptone employed 

 should be as nearly chemically pure as is possible, 

 and indeed the other ingredients should be corre- 

 spondingly free from impurities. Gorini (Centralblalt 

 fur Bakteriohgie und Parasitenhunde, 1893, Bd. xiii. 

 p. 790) calls attention to the fact that impurities in 

 the peptone, particularly the presence of carbohydrates, 

 so interfere with the production of indol by certain 

 bacteria that otherwise produce it, that it is ofttimes 

 impossible, under such circumstances, to obtain the 

 characteristic color-reaction of this body, and where it 

 is obtained it is always after a much longer time than is 

 the case where peptone free from these substances has 

 been used. 



