THE ACTION OF CERTAIN BACTERIA ON SEWAGE 149 
The chemical results in those experiments in which four 
of the bacteria were inoculated into one flask were very ir- 
regular and disappointing. The time of incubation was ap- 
parently insufficient for any one or any combination of the 
bacteria to establish themselves chemically. 
In the experiments in which all of the bacteria were in- 
oculated into one flask there was but one change worthy of 
note. That was the increase in the nitrates in all of the ex- 
periments. This was found under both aerobic and anacrobie 
conditions,—amounting in one instance to six parts per mil- 
lion. There was a decided decrease in the rest of the nitrogen 
present. 
SumMaRY AND CoNCLUSIONS 4 
Certain bacteria have been studied to determine their effect 
upon sewage. Those selected were B. coli, B. cloacae, B. pyo- 
cyaneus, B. vulgaris, B. mesentericus var. ruber, and B. sub- 
tilis. They were selected because they represent aerobic types 
which are found frequently or constantly in sewage. The 
effects of these bacteria were studied in fresh sewage, filtered 
and sterilized by heat at 120° C. (15 pounds pressure) for 
forty-five minutes. They were studied under aerobic and 
anaerobic conditions, also in pure and in mixed cultures. 
Particular attention was paid to the changes in free ammonia, 
organic nitrogen, nitrites and nitrates. 
Bacteriologically it was shown that B. coli was completely 
overgrown under anaerobic conditions in the twenty-four hour 
period. Under aerobic conditions it was able to overgrow B. 
pyocyaneus, but was overgrown by the rest of the bacteria 
studied. 
When grown in pure cultures, with the exception of B. 
proteus, the bacteria gave more constant results under aerobic 
than anaerobic conditions. 
B. subtilis showed a marked ammonifying power through- 
out the work under aerobic conditions. Under anaerobic con- 
ditions it regularly decreased the free ammonia and organic 
nitrogen content, increasing the nitrites and nitrates, especi- 
ally the latter. 
