516 G. J. HTTCKER AND W. A. WALL 



duction. Consequently, the presence or absence of this particu- 

 lar by-product of bacterial metabolism indicates fundamental 

 activities of the bacterial protoplasm. 



In the determination of ammonia production the Thomas 

 (1912) test, which has been used in part of Ayers, Rupp and 

 Mudge (1921), in liquid cultures for studying certain strains of 

 streptococci, has been found to be exceedingly helpful in our 

 laboratory either in the presence or absence of organic matter. 



The organisms to be tested are grown on the following medium: 



Peptone 4.0 per cent 



Glucose 0.2 per cent 



Dipotassium phosphate 0.5 per cent 



Agar 15 .0 grams 



Water 1000.0 cc. 



j,i 



?he culture is incubated at the optimum temperature, pref- 

 erably for one week, and 1 cc. of each of the following reagents 

 added to the surface of the slant : 1 per cent phenol solution and 

 sodium hypochlorite (1 per cent available chlorine). The tubes 

 are allowed to stand for one-half hour and if ammonia is present, 

 a decided blue color appears. 



As an optional method, an adaptation of the common Sorensen 

 method has also proved satisfactory for use with agar slants 

 which contain no peptone or other organic nitrogen. As a 

 reagent, 2 cc. of neutral formaldehyde (containing a few drops of 

 phenolphthalein) are added to each slant. Ammonia production 

 is indicated by the presence of acid which decolorizes the added 

 reagent. The absence of ammonia is indicated by no change in 

 the reaction of the fluid on the surface of the slant. 



Neither of these tests are new ; but their adaptation for use with 

 solid media with free-li\'ing organisms, many of which fail to grow 

 in liquid media, has been helpful in studying their physiological 

 activities. 



These tests are also of special importance in connection with the 

 determination of nitrate reduction, for in some cases a negative 

 nitrite reaction in a nitrate medium does not indicate failure to 

 reduce nitrate. Some organisms may convert the nitrite as 

 rapidly as it is formed either into ammonia or into the protein 



