326 TYPHOID FEVER. 



and becomes thicker. This contrasts very markedly with the 

 colourless film of the B. typhosus. Litmus milk assumes 

 within eighteen hours a marked acidity as shown by the red 

 colour of the medium, and the milk is usually coagulated at any 

 period within four days to one month. Occasionally some 

 varieties are met with which actually fail to cause coagulation. 



The Comparative Culture Reactions of the B. typhosus and 

 the B. coli. The importance of the relationships between the 

 B. typhosus and the B. coli has caused great attention to be paid 

 to their biological characters, in order to facilitate the distinction 

 of the one from the other. Some of these we have already noted. 

 Of the morphological characters the growth on potatoes is the 

 most important. As has been pointed out by Wathelet, and 

 also by Klein, differences exist in the growth of the two bacilli 

 in melted gelatin. A gelatin tube is inoculated, and, instead of 

 being kept at the room temperature, is placed in the incubator 

 at 37 C, at which temperature it is of course fluid. In such 

 cultures, in the case of the B. typhosus, there is a general 

 turbidity of the gelatin, while with the B. coli there are large 

 flocculi developed which float on the surface. It is, however, 

 to physiological differences between the bacilli, rather than to 

 morphological, that importance is to be attached. Several 

 important points are to be studied hereon. 



(i) The Fermentation of Sugars. Chantemesse and Widal 

 were the first to show that the B. coli produced an acid fermen- 

 tation in lactose (milk sugar). Their method was as follows : 

 To tubes of 2 per cent lactose bouillon about I gramme of steril- 

 ised calcium carbonate was added in each case, and the tubes 

 were then sterilised. On inoculating such a tube with B. coli, 

 the acid produced by the fermentation (chiefly lactic acid) acts 

 on the calcium carbonate, setting free bubbles of carbon dioxide 

 which collect on the surface of the liquid. The production of 

 acid in lactose gelatin by the B. coli can also be observed by 

 adding to tubes sufficient blue litmus to make the whole dis- 

 tinctly blue. If a stab-culture be made in such a tube, a red 

 colour diffuses out in the gelatin from the line of growth, and 

 bubbles of gas also form. Later the medium becomes decolor- 

 ised by reduction of the litmus. The addition of lactose 01 other 

 sugars to a simple solution of peptone, however, gives more 

 accurate results (p. 80). The fermentation of lactose by B. coli 



