MICROBIOLOGY OF SEWAGE. 2 19 



habits and functions of the nitrifying organisms. Winogradski 's original 

 types were Nitrosomonas and Nitrobacter, the former oxidizing ammonia 

 to nitrite, the latter completing the oxidation to nitrate. Work upon 

 these organisms constitutes such an important factor in soil bacteriology 

 to-day that more detailed discussion of this nitrifying function is left for 

 another place. 



In the earlier days of sewage purification great stress was laid upon 

 the work of these organisms, which was believed to be fundamental. 

 The degree of nitrification was accepted as a measure of the work of the 

 filters and little thought was given to the possibility of oxidizing reactions 

 by other forms. With the development of modern sewage disposal 

 methods, the work of this latter type of bacteria has assumed a more 

 important role and the actual work of the nitrifying organism has been 

 found to be of only minor and incidental importance. 



Other Oxidizing Reactions. The great groups of aerobic and facultative 

 bacteria are in general concerned in the oxidation of organic matter. 

 There is nothing specific in this reaction and very little that is character- 

 istic of any special or smaller groups. Under certain special and restricted 

 conditions, typical products are formed by particular species, as in the 

 manufacture of vinegar, and it is possible that a careful study of the com- 

 plex reactions involved in the oxidation of sewage would show a certain 

 sequence in the order of events and certain definite work being accom- 

 plished by definite groups. In other words, symbiosis and specializa- 

 tion doubtless take place to a limited extent. But the fundamental fact 

 remains that the metabolism of the organism demands that organic 

 matter be oxidized for the production of energy. Even though certain 

 food substances may be preferred and certain decompositions be norm- 

 ally produced there is necessarily a great latitude and great adaptability. 



For this very reason a study of the individual organism and its action 

 upon specific materials throws no light upon the major problem, which is, 

 given fifty different types of organisms and fifty different fermentable 

 substances, in a mixture, what will be the course of the reaction. Here 

 the preferences, the adaptability and the antagonisms all come into play 

 and while it is impossible to say what has happened or how, it is readily 

 conceived and, in fact, almost apparent, that out of this heterogeneous 

 mixture there will come a homogeneous symbiotic family and an orderly 

 sequence of chemical events, in which metabolic needs and food supply 

 are all delicately adjusted. 



