78 ACTIVITIES OF BACTERIA. 



is red. The method of procedure is self-evident; nutrient 

 bouillon is the medium employed. According to Cahen, 

 all liquefying bacteria reduce litmus. It may be observed 

 very beautifully with, for example, the Bacillus fluorescens 

 liquefaciens. There are also non-liquefying varieties, as, 

 for example, Bact. coli, which present this characteristic. 



2. Reduction of nitrates to nitrites and ammonia. 

 The first property seems to be possessed by bacteria very 

 widely, at least Petri and Maassen found pronounced ni- 

 trite production almost without exception in six varieties 

 grown, upon bouillon containing 2.5% to 5% peptone and 

 0.5% saltpeter ; only once was ammonia alone found. 

 Rubner (A. H. xvi, 53) failed to find nitrite production 

 in isolated instances only; Warington found eighteen pro- 

 ducers of nitrite out of twenty-five varieties. According 

 to our observations, the addition of sugar did not interfere 

 with the process in the case of the Bact. coli, typhi, vul- 

 gare, Bac. anthracis, subtilis, and Vibrio cholerae. After 

 five days upon 0.5% saltpeter, 1% peptone-bouillon, the 

 nitrite reaction was equally great with and without the 

 addition of 1 % of grape-sugar. 



The demonstration of nitrite is conducted as follows: 

 There is added to the nitrate bouillon — also to two uninoc- 

 ulated control tubes — after the tubes have been kept some 

 days in the incubator, some colorless iodid of potassium- 

 starch solution (thin starch paste with 0.5% iodid of potas- 

 sium) and a few drops of dilute sulphuric acid. The control 

 tubes remain colorless, or, at most, gradually become faint- 

 ly blue. If, however, nitrite is present, there occurs a deep 

 blue to (with an abundant amount of nitrite) a dark brown- 

 ish-red color. Small amounts of nitrite are demonstrated 

 with metaphenylendiamin and dilute sulphuric acid (yel- 

 lowish-brown color) or (most distinctly) with a mixture of 

 sulphanilic acid and naphthylamin (red color). Compare 

 Dieudonne (A. G. A. xi, 508). 



The demonstration of ammonia by the addition of 

 Nessler's reagent is only allowable with inorganic and 

 sugar-free nutrient media. In bouillon, almost immedi- 

 ately there occurs a reduction of Nessler's reagent to black 

 mercurous oxid. Strips of paper wet with the reagent 

 may be suspended above bouillon cultures, or the latter 



