108 INFECTIONS OF THE DIGESTIVE TRACT 



work is required to establish the exact position of indol 

 production as a criterion for the classification of bacteria. 

 Unfortunately, nearly all that we know at the present 

 time about the production of indol is based upon the 

 nitroso-indol test, which possesses two grave disadvan- 

 tages. First, this test is not highly sensitive in the detec- 

 tion of indol, and secondly, the test as usually practiced 

 does not exclude the possibility of the presence of other 

 substances which react with indol. I consider it desir- 

 able to replace the old nitroso-indol test by the dimeth- 

 ylamidobenzaldehyde reaction first described by Pro- 

 fessor Ehrlich and which is much more delicate than 

 the nitroso-indol test. For quantitative determinations 

 of indol the yS-naphthaquinone-sodium-monosulphonate 

 method which I have described is unquestionably the 

 best. There is little doubt that in the classification of 

 the intestinal bacteria more and more stress will be laid 

 on the biochemical characters of the organisms in ques- 

 tion. The pathogenicity of an organism is, of course, a 

 highly important character, but is far more variable in 

 general than the biochemical characters just mentioned. 

 Virulent bacteria may easily lose their virulence or have 

 it greatly increased, according to the circumstances under 

 which they grow. The character of the conditions under 

 which an organism has been cultivated must, therefore, 

 be carefully borne in mind in drawing conclusions as to 

 this point. An organism which has long been grown in 

 the laboratory may easily lose its virulence on ordinary 

 media. On the other hand, some microorganisms, 

 such as diphtheria bacilli, may be grown for years on 



