1 20 BA CTEEIOLOG Y. 



menting carbohydrates, their proteolytie function, as 

 evidenced by the appearance of iudol 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. 1 



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 alka- 

 line copper solution. Under the influence of this re- 

 agent pure peptone in solution gives a violet color (the 

 biuret reaction), which remains permanent even after 

 boiling for five minutes. If, instead of a violet color, 

 there appears a red or reddish-yellow precipitate, the 

 peptone should be discarded, as in his experience no 

 indol is produced from peptone giving this reaction. 

 Both the peptone solution and that of the copper (par- 

 ticularly the latter) should be relatively dilute in order 

 for the reaction to be successful. 



LACTOSE LITMUS-AGAR, or LITMUS-GELATIN OF 

 WURTZ. A medium of much use in the differentiation 

 of bacteria is that recommended by Wurtz, consisting of 

 slightly alkaline nutrient agar-agar, to which from 2 to 

 3 per cent, of lactose and sufficient litmus tincture to 

 give it a pale-blue color have been added. Bacteria 

 capable of causing fermentation of lactose when grown 

 on this medium develop into colonies of a pale-pink 

 color and cause, likewise, a reddening of the surrounding 

 medium, owing to the production of acid as a result of 

 their action upon the lactose ; while other bacteria, in- 

 capable of such fermentative activities, grow as pale-blue 

 colonies and cause no reddening of the surrounding 



i See Journal of Experimental Medicine, 1897, vol. ii, p. 549. 



