CHARACTERISTICS OF THE COLON-AEROGENES GROUP 97 
cultures were identical in all their reactions. So far as the 
evidence obtained from these twenty-five cultures goes, there 
1s no relation between the type of colony and the fermenta- 
tion of lactose or the gas ratio. It seems that the tendency to 
‘form outgrowths is an expression of an attempt to push out 
into the medium where conditions are more favorable. Some 
strains have developed this tendency to a greater extent than 
others, but it is very evident that it could not be placed on a 
par with physiological characters for taxonomic purposes. 
The infinity group liquefies gelatin more actively than the 
cloacae group, but its fermentative ability, as shown in Fig. 
5 is less. A comparison based on only twelve cultures must 
be tentative, but some of the differences are of special interest. 
This is the only group in which the correlation between sac- 
charose and raffinose is not nearly perfect. 
The amount of gas formed from dextrose by the infinity 
group is usually much less even than that obtained from 
the low ratio cultures. Traces of gas and a slight change 
in the hydrogen ion concentration sometimes observed in 
lactose broth indicate that there may be a feeble fermentation 
of this sugar by those cultures classed as lactose negative. 
The general characters of the cultures of the infinity group 
identify them with the proteus type. While a few of them 
differ from the usual conception of protcus by fermenting 
lactose, it seems to us that the nature of the fermentation as 
indicated by the end products is so much more basic than the 
nature of the material fermented that the separation into 
species should be made on the gas ratio rather than on the 
failure to ferment lactose. .A number of cultures sent to us 
from other laboratories as proteus failed to ferment lactose 
but gave the carbon dioxide-hydrogen ratio of the aerogenes 
group. It has been customary to include in the colon-aero- 
genes group only cultures that ferment lactose. This is 
largely a matter of convenience and there is no good reason 
why dextrose + lactose — ecnltures should not be included. 
7 
