146 UNIVERSITY OF WISCONSIN STUDIES 
bacteria. This difference was particularly noticeable when 
B. subtilis and B. coli were grown with other bacteria. 
It was to be expected that the chemical results of these ex- 
periments would follow in a general way the influence of the 
predominating bacteria, although the action of the other or- 
ganisms was at times apparent. The time of incubation was 
in some cases evidently too short for the predominating bacter- 
ium to manifest itself chemically. This was plainly evident 
where more than two bacteria were inoculated into one flask. 
In these cases also it is to be noted that B. cola dropped out 
of sight in twenty-four hours in both sets of flasks. In those 
flasks where all of the bacteria were grown together the over- 
growth of B. proteus and B. subtilis was apparent, but it was 
not sufficient to markedly affect the chemical results. A sum- 
mary of the results follows: 
B. coli and B. cloacae.—I have pointed out previously 
that these two organisms have almost. the same action on 
sewage. The results when grown together are quite different 
from what might be expected, especially in the aerobic flasks. 
Here the free ammonia was increased with a corresponding 
decrease in the organic nitrogen. The nitrites were also de- 
creased. No changes were noted in the nitrate content. Ap- 
parently the presence of B. coli was a decided advantage in 
the early part of the incubation period, since it was greatly 
overgrown in twenty-four hours. 
The overgrowth in the anaerobic flasks was probably much 
quicker than in the aerobic flasks, and was much more com- 
plete. The chemical changes noted in this set of flasks are 
those of a pure culture of B. cloacae, the only exception being 
that neither the nitrites or nitrates were attacked. 
B. coli and B. mesentericus var. ruber.—The changes 
here are those due almost entirely to B. mesentcricus var. 
ruber, especially in the anaerobic flasks. 
In the aerobic flasks the small number of B. coli present af- 
fected the results so that they were very irregular. 
B. coli and B. pyocyaneus.—In spite of the fact that 
B. coli overgrew B. pyocyaneus, there were no changes which 
could be attributed to either micro-organism. Evidently B. 
NS 
