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. coli 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. mesentericus 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. 



