INFECTIONS OF THE DIGESTIVE TRACT 107 



and skatol, and to liberate sulphur products such as 

 hydrogen sulphide and methyl mercaptan. The high 

 grade of permanence of the fermentative characters of 

 bacteria give this test much value. This permanence 

 is probably more marked within certain groups such as 

 the colon-typhoid group than in certain others, as, for 

 example, the group of streptococci. Strains of colon 

 bacilli may be carried through a long series of sugar- 

 bouillon fermentation tubes without appreciably modify- 

 ing in a quantitative way their ability to form gas and 

 acid. In a series of experiments in this direction which 

 was carried out by Mr. H. C. Ward, it was found that 

 strains of colon bacilli which had grown for a long time 

 upon a succession of sugar-bouillon tubes grew less well 

 upon peptone bouillon than had previously been the case. 

 On being cultivated in peptone bouillon after a long 

 period of growth on sugar bouillon it was found that they 

 produced only about one-half as much indol as had pre- 

 viously been the case . This result might easily have been 

 interpreted as an evidence that the organisms in question 

 were losing their ability to make indol upon peptone 

 bouillon. Careful examinations of the growths, however, 

 showed that the diminished production of indol was 

 to be referred rather to an impaired ability of the colon 

 bacilli to grow upon peptone bouillon, than to a real 

 inability to make indol. The formation of indol is a 

 character of considerable importance in the classification 

 of bacteria, but there is some evidence that it is not so 

 permanent and invariable a characteristic as the ability 

 of the bacteria to ferment certain sugars. Much more 



