MICROBIOLOGY OF SEWAGE 337 



perhaps most, anaerobes, so the reduction of sulphates, although a 

 less common function, is still common to many forms. In fact ni- 

 trates, sulphates, and phosphates form a series in regard to their 

 reducibility and the effect of their presence upon the reaction as a 

 whole. The phosphates so far as has been recorded are not ordinarily 

 reduced. 



OXIDIZING BACTERIA. The Production of Nitrate and Nitrite. A 

 long series of investigations upon the organisms which oxidize 

 nitrogen began with the Franklands and Winogradski, and has 

 continued to the present day. These have given us much in- 

 formation concerning the 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 facul- 

 tative bacteria are in general concerned in the oxidation of organic 

 matter. There is nothing specific in this reaction and very little that 

 is characteristic 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 care- 

 ful study of the complex reactions involved in the oxidation of sewage 

 would show a certain sequence in the order of events and certain definite 

 work being accomplished by definite groups. In other words, symbio- 

 sis and specialization 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 



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