BACILLI OF THE COLON-TYPHOID-DYSENTERY GROUP 647 



reduction of the neutral-red, decolorize the medium and produce gas by 

 fermentation of the sugar. 



Utilizing the fact that bile-salts are precipitated in the presence of 

 acids, MacConkey devised a medium composed of sodium glycocholate, 

 pepton, lactose, and agar (the composition of this medium is given on 

 page 164), in which Bacillus typhosus grows without causing such 

 change, but distinct clouding results from the growth of the colon 

 bacillus which, producing acid from the lactose, causes precipitation of 

 the bile-salts. 



On Wurtz's lactose-litmus-agar (see page 153) the typhoid bacillus 

 produces no acid, but eventually deepens the purple color to blue; 

 the colon bacillus produces acid and in stab-cultures gas bubbles and 

 the color changes to red. 



In Barsiekow's (see page 164) lactose-nutrose-litmus mixture the 

 typhoid bacillus causes no change, while the colon bacillus produces 

 coagulation and an acid reaction. 



The differentiation of the typhoid bacillus from other similar organ- 

 isms of the typhoid, dysentery, colon group, is based chiefly on growth, 

 upon differential media in which the inability of the typhoid bacillus 

 to form acid or gas from lactose has been the most commonly used basis 

 for differentiation. Various indicators to show whether acid has been 

 formed, added to such media will sharply separate this organism from 

 the colon bacilli and their close relatives. Failure to produce gas with 

 dextrose, differentiates it from the paratyphoid group. The reader is 

 referred to the differential tables given on page 687 and 718, for the 

 basic reactions upon which cultural differentiation is made. The 

 media most convenient for this purpose are, in plates, the Conradi- 

 Drigalski medium, the Endo medium, the Krumwiede brilliant green 

 medium, or the Teague eosin-methylene-blue medium, all of which are 

 described in the section on media; and, in tubes, some of the most 

 convenient media are the Hiss semi-solid mentioned above, Barsiekow's 

 medium, or the Russell double sugar agar. The Russell double sugar 

 agar is particularly useful to give a quick index of differentiation, since 

 it contains both lactose and glucose, and, whereas, the colon group give 

 redness throughout, and a few gas bubbles, the typhoid gives no gas, a 

 red butt due to its action in the depths of the stab on the glucose and 

 an uncolored surface growth. 



Final differentiation is best based upon specific agglutination. 



Winsloir, Klu/lcr and Hothenberg, Jour, of Bacter., 4, 1919, 426. 



