INDOL, PHENOL, ETC. 



6:. 



d 



for five minutes (70° to 80° C). The browniiifj of media due to excess of sodiuiu 

 nitrite must not be mistaken for this pink or red reaction. Uninoculated tubes 

 should be inchided in the test, wliich may be made on the second and tenth day. 



For methods of determining phenol see Lewandowski in Deutsche Med. 

 Wochenschrift, 1890, p. 1186, and Chester's Manual, p. t,;},. Schmidt (Bd. II, 

 p. 1008) gives the following as a qualitative reaction for tyrosiu : Dissolve !:>}' 

 boiling in water and add a solution of mercuric nitrate. The red reaction is 

 sharper if a little fuming nitric acid diluted in water is added. Tr)- also the violet 

 reaction with neutral iron chloride.* Deuciu crystallizes in white soft scales. 



REDUCTION OK NITRATES (aND WORE COMPLEX NITROGEN COMPOUNDS) TO 

 NITRITES, TO AMMONIA, AND TO FREE NITROGEN. 



For the pathologist the iodine-starch reaction is the most satisfactory test for 

 nitrites, because it is not superlatively sensitive and consequently does not indicate 



traces of nitrite absorbed from the air. It is made as 

 follows : Twenty-five cubic centimeters of distilled 

 water are added to one-half gram (more or less) of 

 pure potato starch and the fluid boiled. One cubic 

 centimeter or more of this starch-water and i cc. of 

 freshly prcpai'ed potassium-iodide water (i : 250) are 

 now put into the culture fluid, to which is then added 

 a few drops of strong sulphuric-acid water (2:1). If 

 any appreciable quantity of nitrite is present the culture 

 immediately becomes blue-black from the liberation of 

 free iodine, which acts upon the starch. Old potassium 

 iodide water should ne\'er be used without first testing 

 carefull}', as it usually contains some free iodine. It 

 is always best to first make a trial test without the 

 bacteria. Commercial starch frequently contains prod- 

 ucts of bacterial decomposition and starch prepared 

 aseptically should be substituted. 



At least one-third of the organisms which have 

 " Z) fallen under the writer's observ'ation in recent years 



'^' give the nitrite reaction when grown in peptonized 



beef-bouillon containing potassium nitrate. 



*Mann (p. 323) gives the following as a specific tyrosin reaction: Deniges has recommended 

 the well-known phenol aldehyde reaction for the detection of tyrosin. Nasse, in repeating Deniges' 

 observations, has found the foUowmg to be a very delicate test for tyrosin, as neither proteids 

 nor peptones give the color-reaction. Proceed thus; Add a few drops of formol solution to con- 

 centrated sulphuric acid, when, on warming with tyrosin, a brown-red color is obtained, which, on 

 addition of acetic acid, becomes green. 



fFiG. sd.—Bacicnum syriiigae (van Hall). Nitrate bouillon cultures 5 days old, to each of 

 which has been added boiled starch water, potassium iodide water, and sulphuric acid. In tube a 

 the potassium nitrate was reduced to the nitrite, and on addition of the reagents free iodine was 

 liberated and the starch blued. In the other no nitrite had formed, no iodine was liberated, and the 

 starch remained colorless. For discrepancy see te.xt. 



