CHARACTERISTICS OF THE COLON-AEROGENES GROUP 79 
tures used consisted of a collection of 689 cultures fermenting 
dextrose with evolution of gas, without spore formation, 
with abundant growth on agar and diffused growth in broth. 
One hundred and fifty-two of these came from 27 samples of 
human feces, 148 from 17 samples of bovine feces, 132 from 
31 samples of water, 143 from 33 samples of grains, and 114 
from milk. They were obtained for the most part by direct 
plating on asparagin lactose litmus agar followed by replating 
and a test for fermentation in dextrose or lactose broth. 
THE FORMATION OF GASES 
The active evolution of gases, indicating a more profound 
fermentation of the carbohydrates, is the striking character 
which distinguishes the colon-aerogenes group from most of 
the carbohydrate-fermenting bacteria. The nature and the 
relative proportion of gases evolved has come to be regarded 
as of little significance and the fermentation tube of Theobald 
Smith is now used merely to determine the fact of gas pro- 
duction. The gas ratio, proposed by Smith? as a diagnostic 
character, has fallen into disrepute through its failure to 
correlate with habitat or with other physiological characters. 
This attitude is well illustrated by the paper by Longley and 
Baton. The probable cause of this failure is found in the 
work of Keves* who pointed out the great variation in the 
gas ratio as ordinarily determined and its constancy when 
determined by accurate methods. Our results have confirmed 
the conclusions of Keyes in every way and show that the 
nature and amount of the gaseous by-products formed under 
uniform conditions are remarkably constant. 
The same culture, or even different cultures of the same 
variety, will repeat the ratio of carbon dioxide to hydrogen 
with almost mathematical accuracy. This is illustrated by 
2Theobald Smith, The Fermentation Tube with Special Reference to 
Anaerobiosis and Gas Production among Bacteria in Wilder Quarter-Cen- 
tury Book, pp. 187-232, 1893. 
3F. F. Longley and W. U. C. Baton, Notes on the Determination of B. 
Coli in Water in Jour. Inf. Dis., 4, 3, pp. 397-416, 1907. 
4 Frederick G. Keyes, The Gas Production of Bacillus Coli in Jour. Med. 
Fess 21, 1, We 69-82, 1908. 
