38 BACTERIOLOGY 



to carry hydrogen sulphide to higher sulphur compounds, 

 the so-called "thiogenic" species. 



Those saprophytes that are concerned in such well-known 

 fermentations as result in the production of the various 

 acids of the fatty series are known as "zymogens." 



But of all the so-called saprophytic group none are more 

 interesting and none by any means so important as those 

 concerned in the various transformations through which 

 nitrogen passes in being prepared as food for higher vege- 

 tation. This group, or rather those groups, for there are 

 apparently several operating on nitrogen and its compounds 

 in various ways, are known as the "nitrifying" and the 

 "denitrifying" and the " nitrogen fixing" bacteria. 



NITRIFYING BACTERIA. They carry ammonia, resulting 

 from the decomposition of dead animals and plants; by 

 a process of oxidation first to nitrous acid, then by further 

 oxidation the nitrous acid is carried to nitric acid. These 

 two steps in the process are taken by two totally distinct 

 groups of bacteria of a most interesting nature. The func- 

 tion of one group is strictly limited to the nitrite process; 

 that of the other to the nitrate; the latter taking up the 

 work at the point where the former leaves it. The former 

 cannot carry its operation beyond the nitrite point, nor can 

 the latter begin with ammonia and carry it to complete 

 nitrifaction. A most singular peculiarity of this group is 

 the inability to develop on the nutrient media commonly 

 used for the cultivation of bacteria. Organic matter as 

 such seems to be unfavorable to their viability. To grow 

 them one is obliged to use a silicate jelly, a sort of water 

 glass of about the consistency of ordinary gelatin, to which 

 are added certain salts that these particular species are 

 able to decompose in order to secure the elements necessary 



