GROWTH 169 



In comparing nitrates with ammonia, it is customary to add to the 

 same mineral-sugar solution in one case a nitrate (KNO3 or NaNOs 

 most commonly) and to the other solution an ammonium salt (NH4CI 

 or (NH4)2S04). If the mineral solution had been a balanced salt 

 solution, the addition of a potassium salt might make it slightly 

 poisonous; if it had been unbalanced, the potassium salt might balance 

 it more perfectly and cause a better growth. The same explanation 

 probably accounts for the fact that some investigators find Ca(N03)2 

 to produce a much better growth than KNO3 while others find the 

 opposite. It is not very probable that the ion of a soluble ionized 

 salt has different food value because it was originally combined 

 with a different base. 



Nitrates and especially ammonia are for some bacteria 

 the only source of nitrogen, and organic nitrogen com- 

 pounds cannot be utilized by them. In this group we 

 find the organisms with a very simple, mineral form of 

 fermentation, such as the nitrifying organisms, the 

 sulphur bacteria, and, perhaps, the iron bacteria. They 

 are the same species which can build their cells only from 

 CO2 and not from organic compounds. Nitrate-assimi- 

 lating bacteria are known in great numbers. The 

 denitrifying bacteria, Ps. fluorescens, Ps. pyocyanea, B. 

 suhtilis are able to live on nitrates and some nitrogen- 

 free organic matter. In soil, such species are found 

 abundantly. Some representatives of the colon group 

 can live with nitrate as the only source of nitrogen 

 (Kisch, 1919). 



Ammonia is also used commonly (e.g., by the colon- 

 paratyphoid group, and by many soil bacteria) and, 

 perhaps, more commonly than nitrates. It is a fairly 

 good source of nitrogen for yeasts which refuse nitrates 

 as a rule (Lindner, 1905). Many of the bacteria which 

 use nitrates, can also use ammonia, though some excep- 

 tions are mentioned. In experiments with ammonium 

 salts, it must be kept in mind that the consumption of 



