[V or. 7 
280 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
Rupp have made quantitative determinations of formie, acetic, 
lactic, and succinic acids from Bacillus coli in a dextrose bouillon, 
and Wyeth, and Cohen and Clark have shown that the critical 
hydrogen ion concentration varies with the different acids, 
hydrochloric, acetie, and lactic, indicating that the anions of 
the acids or perhaps the undissociated molecules, as Winslow 
and Lochridge suggested, are also concerned in inhibition. Most 
of the work on the inhibitory effect of different acids has been 
based on inoculation of media of different Pa values obtained 
by using different acids, and the inhibition has been determined 
according to the presence or absence of growth after a certain 
interval. Such a method does not take into consideration 
milder phases of inhibition which are not severe enough to cause 
the death of the organisms. To illustrate this phase and to 
indieate some of the relations of the hydrogen ion factor to 
the other factors, the results of an experiment are presented in 
table x and fig. 10. Culture 1 was grown in 1 per cent dextrose 
bouillon, and Culture 2 in plain bouillon to which sterile N /5 
HCI was added, as indicated in table x, in an attempt to simu- 
late in plain bouillon the P, curve of a culture fermenting dex- 
trose bouillon, such as Culture 1. As seen from fig. 10, the 
culture produced alkali continually so that it was only possible 
by frequent additions of acid to hold the P, in a zone around 
P, 4.8, the greatest hydrogen ion concentration which Culture 1 
attained. The growth curve, fig. 10, shows marked acid inhibi- 
tion with almost no further inerease in growth after the first 
addition of acid at 14 hours. There is practically no difference 
in the growth curves of the two cultures up to 72 hours, but 
from that point they separate widely, for death occurs shortly 
in the dextrose media and growth in Culture 2 does not go below 
26,000,000 bacteria per cc. Thus a hydrogen ion concentra- 
tion of P, 4.8-4.9 when produced by the acid fermentation of 
dextrose was fatal, while that of Pa 4.7-5.1 from HCl was 
only strongly inhibitory, indicating that the other metabolic 
products of dextrose fermentation, such as acetate or lactate 
ions, evidently enter as factors in causing the death of the culture. 
