2l8 MICROBIOLOGY OF WATER AND SEWAGE. 



alter the course of the other reactions involved. The products of the 

 protein hydrolysis for example may be profoundly modified by the pres- 

 ence of this additional source of oxygen. 



The effect upon the bacteria themselves is also to be considered as a 

 factor quite distinct form the purely chemical effect just described. It 

 has frequently been observed, and in fact would be expected, that the 

 products of anaerobic putrefaction are themselves detrimental to the activ- 

 ity of the organism producing the changes in question. The nature of 

 sulphuretted hydrogen makes it appear quite probable that we are dealing 

 here with a toxic substance that would at least inhibit the activities of 

 certain bacteria and in this way further modify the final result. 



The same might be said of almost all the reactions with which we 

 have to deal but this example is cited as a typical one. 



It is known in practice that the presence of sulphates in a sewage does 

 lead to a distinct type of anaerobic change which is characterized by the 

 marked blackening of the sewage, the formation of secondary reaction 

 products which precipitate after the removal of the suspended matter of 

 the sewage, the evolution of hydrogen sulphide, an excessive amount of 

 mineral or non-volatile residue in the sludge and the formation of free 

 sulphur upon subsequent aeration of the sewage. 



Here again, as in the other types of reaction, it is useless for the present 

 to attempt to ascribe this reaction to any particular species. Sp. 

 desulphuricans and B. sulphur eus have been isolated. A non-liquefying 

 anaerobic bacillus, which reduced sulphates strongly, was isolated from 

 Boston sewage in the writer's laboratory by G. R. Spaulding. Others have 

 been described and there is undoubtedly a large group of organisms capable 

 of bringing about the reaction. 



Just as the reduction of nitrates is a function performed by many, 

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

 common function, is still common to many forms. In fact nitrates, 

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

 and the effect of their presence upon the reaction as a whole. The phos- 

 phates 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 information concerning the 



