128 BACTERIOLOGY 



It is usually of a neutral or slightly alkaline reaction 

 and neutralization is not, therefore, necessary. It is filtered, 

 decanted into tubes or flasks, and sterilized 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 consideration possesses the property of producing 

 indol as one of its metabolic products. It is essential for 

 accuracy that the preparation of dried peptone employed 

 should be as nearly chemically pure as is possible, and indeed 

 the other ingredients should be correspondingly free from 

 impurities. Gorini^ 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 pep- 

 tone free from these substances has been used. 



Peckham has also demonstrated that where bacteria 

 have the property of forming indol and also of fermenting 

 carbohydrates, their proteolytic function, as evidenced by 

 the appearance of indol as a product of metabolism, may be 

 completely suppressed by the addition of such fermentable 

 carbohydrates as glucose, saccharose, and lactose to the 

 proteid solution in which they are developing.^ 



Gorini suggests the advisability of testing the purity of 

 all peptone preparations before using them, by means of 

 the reaction that they exhibit with Fehling's alkaline copper 

 solution. Under the influence of this reagent pure peptone 

 in solution gives a violet color (the biuret reaction), which 



' Centralblatt flir Bakteriologie und Parasitenkunde, 1893, vol. xiii, p. 790. 

 - See Journal of Experimental Medicine, 1897, vol. ii, p. 559. 



