REACTIONS OF B. TYPHOSUS AND B. COLI 327 



peptone-salt glucose, lactose and mannite, but not in cane sugar, 1 

 while the b. typhosus produces acid without gas in bile-salt 

 glucose, peptone-salt glucose, and mannite, but not in lactose or 

 cane sugar. It can also cause similar changes in arabinose, 

 galactose, and fructose. 



Gas production by the b. coli can also be demonstrated 

 by means of shake cultures. As ordinary bouillon contains 

 traces of glucose it is best to use peptone-salt solution to which 

 an appropriate sugar has been added and which has been 

 converted into a solid medium with 10 per cent gelatine. If 

 such a medium be inoculated in the fluid condition, shaken and 

 set aside till growth occurs, small bubbles of gas will form all 

 throughout it. In ordinary media inoculated with the b. coli 

 bubbles of gas are often developed along the needle track. 



In the case of acid production by the b. coli or b. typhosus in 

 ordinary media the acid probably comes, as has been said, from 

 the glucose developed from the muscle sugar, but there may also 

 be a subsidiary acid formation from the breaking up of the 

 proteid elements. 



In certain members of the coli-typhoid group it has been 

 observed that in such media as litmus milk or litmus whey an 

 acid reaction may be first produced, and this may be followed 

 after a few days by the formation of alkali, and in certain cases 

 this phenomenon may be helpful in differentiating the species. 



Curdling of Milk by the B. coli. This probably depends on 

 the fermentation of the lactose of the milk and the throwing 

 down of the casein by the resulting lactic acid ; but the reaction 

 may be a more complicated one, as milk can be curdled by 

 organisms which do not possess acid-forming properties. In any 

 case the observation of the reaction is important. The typhoid 

 bacillus produces no visible change in milk. 



(2) Action on Media containing Neutral-Red. While, as will 

 have been already gathered, neutral-red is used as an indicator, 

 there is some evidence that an actual breaking up of the 

 substance can take place by the action of the coli-typhoid group ; 

 the evidence for this lies in the fact that when the effects of 

 acid formation are observed the tint of the medium cannot be 

 brought back by the addition of alkali. The medium used here is 

 bouillon containing an appropriate sugar and *5 per cent of a 1 

 per cent watery solution of Grubler's neutral-red. In the case of 

 the typhoid bacillus no change occurs, but in the case of the b. 

 coli there is developed a beautiful canary yellow with a greenish 



1 A variety of the organism which does ferment 'cane sugar has been 

 described under the name of the b. coli commuuior. 



