152 MICROBIOLOGY 



Nitrifying bacteria. According to recent observations, 

 nitrification is produced by a small group of bacteria which 

 are cultivated with difficulty. Hellriegal and Welfarth and 

 many other investigators in this field have succeeded in dem- 

 < nstrating that there are certain bacteria which possess the 

 power of taking nitrogen from the air and making it avail- 

 able for plant food. In this process, there appear to be two 

 'distinct species present in the soil, one of which oxidizes am- 

 monia to nitrites and the other converts nitrites to nitrates. 



Conversion of nitrous and nitric acids into free oxygen. 

 Barrie and Stulzer seem to be the first who accurately de- 

 scribed the nitrate-fermenting bacteria. In their investiga- 

 tion they isolated from horse manure two bacteria, neither of 

 which was alone capable of producing nitrogen from nitrates 

 but which together in the presence of oxygen decomposed ni- 

 trates. Later a* second de-nitrifying bacillus was found, 

 J?. denitrificans II, which would produce nitrogen from ni- 

 trates. 



Nitrogen combinations. Bacillus radiocola, isolated by 

 Beyerinck, has the power of assimilating nitrogen from the 

 air. This bacillus is found in the small root nodules of various 

 leguminous plants (peas, clover, etc.) and can be obtained 

 m cultures from the nodules on the roots of these plants. Dif- 

 ferent varieties are said to exist in different species of legumes, 

 each kind of legume apparently having a special kind of bac- 

 teria adapted to it, as every variety is not capable of produc- 

 ing nodules in all legumes. There are certain ^ neutral" 

 varieties, however, existing free in the soil and not adapted 

 to any special legume, and these seem to be able to form 

 nodules in the different species. 



Production of acids. Many bacteria are able to form 

 free acids in culture media containing sugar. Acid is formed 

 in ordinary bouillon because of the presence of dextrose which 

 usually occurs in small quantities in the meat. According to 

 Theobald Smith, all anaerobic or facultative anaerobic bacteria 

 form acids from sugar ; the strict aerobic species do not, or so 

 slowly that the acid is concealed by the almost simultaneous 



