18 North Carolina Agricultural Experiment Station 
In conclusion, the present studies appear to be in accord with several others 
previously discussed in showing that the activity of the hydrogen ions alone 
is not responsible for the inhibitory effects which have been observed. This 
is opposed by the conclusions of a number of investigations, among which are 
those of Norton and Hsu (5), who inclined to the view that the disinfecting 
power of an acid is approximately proportional- to the hydrogen ion concen¬ 
tration. Whatever may be the ultimate solution of this problem, it is apparent 
that further developments will depend upon the devising of methods which 
will show what are the dissociation products of the various acids in such 
complex solvents as bouillon or agar and whether or not these dissociation 
products in the presence of the H-ions modify the permeability of the bac¬ 
terial cell wall.* 
*Tlie complexity of the problem of the influence exerted by the hydrogen ion 
on bacterial inhibition is further demonstrated by the voluminous literature 
in the field involving a study of antiseptics and by several bacteriological pa¬ 
pers which either were not available when this manuscript was submitted or 
appeared while it was in press. It is entirely outside the scope of the present 
paper to summarize the work on the H-ion and its relation to inhibition and 
antisepsis. A brief statement regarding a few papers will suffice to show this 
relationship and to guide the reader to a proper appreciation of this problem. 
Friedenthal (1) in a study of the comparative antiseptic power of variouk 
substances and ionic groups noted that no ion is of such powerful antiseptic 
virtue as H-f, one gram of which was computed to be capable of maintaining 
sterility in 30,000,000 liters of culture medium. Paul and Kronig (2) noted 
that the toxicity of metallic salts for Bacillus anthracis and Staphylococcus 
aureus depends not only on the cation but on the anion and undissociated 
molecules as well. The same idea is maintained in later work by Paul and 
his associates in two papers which appeared in 1910 (3 & 4). In addition, it 
is pointed out that the hydrogen ions of weak organic acids appear to be cata¬ 
lyzed by the anions and that neutral salts, when added to inorganic acids, are 
capable of increasing the toxicity of these acids. 
In a comparative study of the standardization of disinfectants by the Rideal- 
Walker and the Hygienic Laboratory methods, Wright (5) found that the dis¬ 
crepancies noted by himself and others were due in part to variation in the 
hydrogen ion concentrations of different batches of media. In addition, the 
pH value of the Rideal-Walker broth was approximately 7.65, while that of 
the Hygienic Laboratory was 5.55; media so different could not be expected to 
give comparable results. 
The several bacteriological investigations to which reference was made are 
based upon a “physiological constant” postulated by Michaelis and Marcora 
in 1912 (6). They showed that B. coli ceased the production of acid in a 
lactose bouillon at a concentration of Ixl0-5N, which point they considered to 
be a physiological constant for this organism. Several factors, however, serve 
to modify the final reaction at which activity ceases. Clark (7) found that B. 
coli from bovine feces brought a 1 per cent peptone plus 1 per cent dextrose to 
pH=4.26, while with 1 per cent lactose only pH=4.56 was reached. He also 
noted distinct differences in the final reaction with the same organism in dif¬ 
ferent media, i. e., the greater the buffer effect of the medium, the lower the 
final H-ion concentration attained. The results of studies by Wolf and Harris 
(8) show that the final reaction varies dependent on two other factors, namely : 
the reaction of the medium before inoculation and the kind of acid used. With 
Welches’ bacillus in 2 per cent peptone plus 2 per cent glucose broth, when ad¬ 
justment is made with HC1 to an initial reaction of pH 6.77 growth ceases 
when sufficient acid is produced to give the medium a reaction of pH 4.59. 
When the initial reaction was pH 4.86 a final reaction of pH 4.34 was attained. 
Initial reactions between these two extremes give final reactions which take up 
orderly positions between pH 4.59 and pH 4.34, showing that there is a defi¬ 
nite relation between initial and final reaction. The results with B. sporogenes 
