THE CHEMISTRY OP BACTERIA. 109 



It may be of interest to note that nitrites are almost al- 

 "ways present in the saliva, as a result of the action of the 

 mouth bacteria upon the food constituents. Nitrates do not 

 seem to be present. 



Among the nitrifying organisms there are some which 

 differ markedly from all other known bacteria. Unlike the 

 latter, they do not require organic matter inasmuch as they 

 . are able to assimilate carbonic acid. Without the aid of 

 chlorophyll, and without light, they can utilize the carbonic 

 of the air as food. Not only is organic matter unnecessary, 

 but it is even injurious, and retards the growth of these 

 bacteria. Hence, special methods, wholly unlike those or- 

 dinarily employed when cultivating bacteria, must be re- 

 sorted to, in order to isolate these remarkable organisms. 

 They can be grown best in pure water to which 0. 1 per cent, 

 of ammonium sulphate and of potassium phosphate, and 

 some basic magnesium carbonate has been added. They 

 can also be isolated by growing upon a nitrite-agar, or upon 

 a mineral-gelatin which is essentially silicic acid. 



In view of the cultural requirements of these organisms 

 it will be seen that they belong to the simplest and earliest 

 forms of life. They exist on wholly inorganic matter, since 

 the carbonic acid of the air supplies the necessary carbon, 

 while ammonia yields the nitrogen essential to the forma- 

 tion of protoplasm. These organisms convert the ammonia, 

 by process of oxidation, first to nitrous acid and then to 

 nitric acid. These changes can be indicated by the equa- 

 tions : 



2 NHj -f 3 O2 = 2 HNO2 + 2 HjO. 



2 HNOi, + O2 = 2 HNO3. 



While there are nitrifying organisms which can change 

 ammonia into nitrates, there are many others which can 

 produce nitrites, but not nitrates. Other organisms, in that 

 case, complete the process, converting the former com- 



