[Vor. 7 
286 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
trolled to à certain extent by the hydrogen ion concentration, 
which can in turn be controlled by the amount of dextrose fur- 
nished. The initial amount of dextrose determines the amount 
of acid produced or the maximum hydrogen ion concentration 
attained. 'The work of Clark and Lubs, Besson, Ranque and 
Senez, and the experimental data presented here give a rather 
definite idea of the action of Bacillus coli according to the amount 
of dextrose in the medium. With .3 per cent or less of dex- 
trose, insufficient acid is produced to kill the organisms; .4 per 
cent or more is sufficient dextrose to produce acid to Py 4.9 or 
better, and the culture becomes sterile in 6 days or less. An 
&mount of dextrose not accurately determined, but between 
.3 and .4 per cent, probably depending to some extent on the 
buffer in the medium, should produce just enough acid, between 
Pa 5.1 and Py 4.9, depending on the time of exposure, to kill 
the culture. If insufficient acid to kill the culture is produced, 
as from .3 per cent or less of dextrose, a reversion of reaction 
takes place, which Ayers and Rupp have explained with Bacillus 
aerogenes as the formation of alkaline carbonates from the or- 
ganic acids, especially from the formic and acetic acids. There 
is a similarity in reaction and in growth curves between Bacillus 
aerogenes and Bacillus coli, the main difference appearing to 
be in the greater acid resistance of Bacillus aerogenes. Growth 
in the cultures where reversion of reaction takes place seems 
to be typical. One-tenth per cent of dextrose provides a stimu- 
lation to growth, but greater amounts produce some evidence 
of acid inhibition, followed by an increase in growth with the 
reversion of the reaction and alkaline inhibition between Py 7.0 
and 7.6. The least inhibition is found in a culture in which the 
hydrogen ion concentration is held in a narrow zone around 
the neutral point—probably P, 6.0-7.0 is the best—by adding 
small amounts of dextrose at frequent intervals. Thus, with 
Bacillus coli, hydrogen ion concentration and growth within 
limits can be manipulated by the dextrose furnished. The 
growth curves emphasize not only the value of the initial reac- 
tion and composition of the medium, but also the importance 
in physiological studies of following the changes in hydrogen 
ion concentration which the growing bacteria produce in their 
substrates. 
