112 BACTERIAL POISONS. 



duces 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 present, gives the 

 coloration. 



From a very exhaustive 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 action of the bacteria on 

 nitrates. The reaction is most marked at blood -tempera- 

 ture 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 

 nutritive solution contains 0.01 per cent, of nitrate. 



(6) The reaction is most marked if the sulphuric acid be 

 added after the addition of a very dilute 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. GRIESINGER, in 1885, 

 also made mention of the production of a red coloration in 

 rice-water stools on the addition of nitric acid. 



A " cholera-blue " has also been observed by BRIEGER 

 in cultures in meat extract containing peptone and gelatin. 

 This substance, which is yellow by reflected, and blue l>y 

 transmitted light, is developed by the addition of concen- 

 trated 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 hydrate, 

 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 



