60 BACTERIAL POISONS OF INFECTIOUS DISEASES. 



reaction by supposing that the germ produces nitrous acid, which 

 exists in the culture as a nitrite ; on the addition of an acid the 

 nitrous acid is set free and acting upon the indol, which is also pres- 

 ent, gives the coloration. After all, wider experience has shown 

 that Cohen was right and that many germs respond to the indol 

 reaction. 



According to Hammurl, cultures of the germ newly taken from 

 stools or intestinal content do not give the indol reaction as con- 

 stantly as those germs grown on artificial media. From an ex- 

 haustive research on the importance of this test, Petri comes to the 

 following conclusions : (1) Seven pure cultures of the cholera germ, 

 from as many sources, gave the reaction with equal distinctness. 

 (2) Of one hundred other bacteria tested in the same way, twenty 

 gave a red coloration. In nineteen of these the coloration is due to 

 the nitroso-indol reaction of Baeyer. The twentieth (anthrax) gave 

 a color which is not due to indol. (3) In case of the cholera germ, 

 and the others as well, the reaction is due to the reducing effect of 

 the bacteria on nitrates. It is most marked at blood temperature 

 and with the cholera bacillus ; it is least distinct with the bacilli of 

 Finkler and Miller. (4) None of these bacteria convert ammonia into 

 nitrite. (5) The simple addition of sulphuric acid is sufficient to give 

 the test, which, however, is most marked when the nutrient solution 

 contains 0.01 per cent, nitrate. (6) The reaction is most marked if 

 the sulphuric acid be added, after the addition of a very dUute 

 nitrite solution. 



Schuchardt calls attention to the fact that Virchow observed a 

 red coloration on the addition of nitric acid to filtered cholera stools 

 in 1846, and in 1885 Griesinger made mention of a similar obser- 

 vation. 



A " cholera blue " has also been observed by Brieger in cultures 

 in meat extract containing pepton and gelatin. This substance, 

 which is yellow by refracted, and blue by transmitted, light, is de- 

 veloped by the addition of concentrated sulphuric acid to the culture. 

 It may be separated from the " cholera red " as follows : Treat the 

 culture with sulphuric acid, then render alkaline with sodium hy- 

 drate and extract with ether. Evaporate the ether and remove the 

 " cholera red " with benzol, then again dissolve the " cholera blue " 

 in ether. The characteristic absorption bands for this coloring 

 matter begin in the first third of the spectrum between E and F and 

 darken all of the zone lying beyond. 



Winter and Lesage treat a bouillon culture of the cholera germ 

 with sulphuric acid, dissolve the precipitate in an alkaline medium, 

 reprecipitate with acid and redissolve in ether, which on evaporation 

 leaves oily drops, and these, on cooling, form a yellow mass of the 

 appearance of a fat. This substance is insoluble in water and acids, 

 soluble in alkalis and ether. It melts at 50°, and does not lose its 



