CHARACTERISTICS OF THE COLON-AEROGENES GROUP 103 
B. cloacae may be considered as the stem from which the 
other members of the group have sprung. By its vigorous 
habits of growth, the wide range of substances from which 
it is able to obtain its supply of carbon and its ability to ex- 
crete proteolytic enzymes, it is able.to thrive under the sapro- 
phytic conditions in which it is found. B. aerogenes has lost 
the proteolytic ability and acquired semi-parasitic habits. 
B. coli is still more parasitic in its habits, is more restricted 
in its range of carbon supply, and is further removed from 
B. cloacae and B. aerogenes by the loss of the carbinol reaction 
and by the development of the ability to form indol. Still 
more parasitic in their habits and correspondingly more re- 
moved from B. cloacae by their physiological characters are 
B. enteritides, B. typhosus and B. dysenteriae. Removed 
from B. cloacae in the opposite direction is B. proteus which 
has developed the proteolytic ability but lost in the utilization 
of carbohydrates both in the range of materials fermented and 
in the extent of the fermentation. 
