CHARACTERISTICS OF THE COLON-AEROGENES GROUP 85 
son™ shows that the acids formed by the accepted types of 
B. coli, and B. aerogenes are identical but that the amounts 
of these acids produced by B. coli are less than those pro- 
duced by B. aerogenes. The results obtained by Ayers and 
Rupp show that the final difference is due primarily to dif- 
ferences in the rates of the various fermentations. In the 
low ratio type the acid formation proceeds faster than the 
fermentation of the acid salts and a hydrogen ion concentra- 
tion which inhibits growth may be reached. In the high ratio 
group the fermentation of the acids is more rapid and the 
initial acid reaction may be converted into an alkaline one. 
With these facts before us it is easier to explain the more 
complicated and more variable gas evolution of the high ratio 
group. 
By carefully adjusting the amount of sugar and the buffer 
action, Clark and Lubs?? devised a medium in which the low 
ratio cultures reached the limiting hydrogen ion concentra- 
tion while the high ratio cultures fermented the greater part 
of the sugar without reaching this point, thus permitting a 
reversion of the reaction toward the alkaline side. The Py 
value of 5.0 — 4.8 (Sorensen’s scale), reached very uniformly 
by the low ratio group, gives a red color with methyl red 
while the high ratio cultures are uniformly yellow. Nearly 
all of our cultures have been subjected to this test and it has 
been found to agree with the gas determination in every case. 
With only a few cultures was there any question as to the 
reaction and in these cases it was found that there was some 
abnormality in the gas ratio which would justify classing 
these cultures as atypical. Other workers who have used 
this test have also found it a reliable method of distinguishing 
between the B. coli and B. acrogencs type, but some, who have 
considered it less reliable than the Voges-Proskauer test, 
have apparently failed to realize that it was designed to in- 
11 James Thompson, The Chemical Action of Bacillus Cloacae (Jordan) 
on Glucose and Mannitol in Proc. Roy. Soc. (London, 1912), Ser. B, 84, 
pp. 500-504. 
"122°.W. M, Clark and H. A. Lubs, Improved Chemical Methods for Differ- 
entiating Bacteria of the Coli-Aerogenes Family in Jour. Biol. Chem., 
30, 2, pp. 209-234, 1917. 
