CHARACTERISTICS OF THE COLON-AEROGENES GROUP 83 
in the gas produced by the low ratio group, a similar change’ 
in the composition of the medium materially affected the gas 
ratio of the high ratio group by increasing the CO,. The 
possibility of distinct fermentations, one producing CO, 
and H, in equal parts and one producing CO, independently 
of the hydrogen, is strengthened by the fermentation observed 
in the third group which gives carbon dioxide only and for 
which the ratio may, for convenience, be designated by the- 
«sign. Only twelve cultures of this type were found, but 
the unusual end product of their fermentation entitles them 
to consideration as a separate type. 
The statement by Burton and Rettger® that we believe that 
the two reactions are mutually dependent does not express the 
opinion we have held on this subject. On the contrary we 
have found considerable evidence to lead us to believe that 
these fermentations were independent. Burton and Rettger 
have taken a similar view and have elaborated an hypothesis 
to account for. the two fermentations. They assume that in 
one fermentation nearly equal volumes of carbon dioxide 
and hydrogen are produced with sufficient acidity to suppress 
the second fermentation when the former predominates. In 
the second fermentation butylene glycol is fermented with 
the formation of considerable quantities of gas and acetyl- 
methyl-carbinol, the substance giving the Voges-Proskauer 
reaction. Since the end products of the reaction are not acid 
in their character, a proteolysis is permitted and sufficient 
ammonia to account for the alkaline reaction sometimes ob- 
served is produced. 
CORRELATION OF OrHER CHARACTERS WITH THE Gas Ratio 
The ‘‘Methyl red test.’’—It has been shown by Michaelis 
and Marcora’ that the growth of a colon culture ceases when 
a certain hydrogen ion concentration is reached. This re- 
‘lL. V. Burton and L. F. Rettger, Correlations in the Colon-Aerogenes 
Group in Jour. Inf. Dis., 21, 2, pp. 162-195, 1917. 
7L. Michaelis and F. Marcora, Die Stuerproduktivitit des Bacterium 
Coli in Ztschr. f. Immunvititsforsch. uw. exper. Therap., Orig., 14, 2, pp. 
170-173, 1912. 
