150 UNIVERSITY OF WISCONSIN STUDIES 
B. coli and B. cloacae had a decided reducing action on 
both the nitrites and nitrates. Under some conditions, as yet 
undetermined, they reduced the free ammonia content also, 
B. proteus had a considerable ammonifying power under 
anaerobic conditions. This was very slight under aerobic 
conditions, and was more constant at room temperature than 
at 37.5° C. 
The action of B. pyocyaneus and B. mesentericus var. ruber 
was irregular under both aerobic and anaerobic conditions. 
The experiments with mixed cultures showed that the 
chemical changes followed very closely those of the predomi- 
nant bacteria in pure culture. Those experiments which did 
not follow this rule, and in which there was a decided pre- 
dominance of one micro-organism, showed that the bacteria 
had become predominant too late in the incubation period to 
effect a chemical change. 
B. coli and B. cloacae, B. coli and B. proteus gave more 
constant results when grown in mixed than when grown in 
pure cultures. These were the only instances of an appar- 
ently true symbiotic relationship. 
The results of the experiments in which more than two 
bacteria were grown in mixed culture showed that the in- 
cubation period, twenty-four hours, was too short to allow any 
one micro-organism of any group of bacteria to establish it- 
self chemically. 
The work has shown that we will be able to predict what 
the changes in the chemical composition of sewage are going 
to be if a group or species of bacteria are predominant. 
The results of this work would certainly not justify us 
in attempting to control the bacterial flora of the septic tanks. 
However, I am strongly convinced that in the future this 
course will be attempted. 
I again wish to emphasize the results obtained from B. 
subtilis, Everything indicates that this organism may be 
made to play a most important réle in the treatment of sew- 
age. 
