Studies on Physiology of Some Plant Pathogenic Bacteria 
15 
After dissolving the materials, the media were neutralized, flashed in 200 cc. 
quantities and sterilized. As soon as they had cooled to about 60°C. commer¬ 
cially pure concentrated acid of the appropriate kind was added, using aseptic 
precautions, to obtain the desired pH values. The pH values were determined 
colorimetrically by comparison with buffer color standards. These buffer so¬ 
lutions were checked electrometrically and were found to be less than their 
assigned value by .03-.06 pH. After the medium had been adjusted to the de¬ 
sired H-ion concentration, it was poured into sterile test tubes. Such a pro¬ 
cedure as demonstrated in a previous paper (1) is of especial value in the 
preparation of solid media since it removes the necessity of sterilization after 
adjustment of reaction and thus does not modify the acidity nor destroy the 
jellifying power. All tubed media were incubated for 48 hours before inocu¬ 
lation to determine their freedom from contamination. 
RESULTS 
Growth in bouillon, as shown in Table 1, was determined by the ability of 
the several organisms to cloud the media. In the case of abundant clouding, 
the sign + is employed, no visible growth is indicated by—and a slight cloud¬ 
ing by-h 
Manifestly the H-ion concentration alone does not determine the limit of 
tolerance of these organisms since one would then expect irrespective of the 
acid, cellular multiplication to be checked at the same point. The several or¬ 
ganisms are seen to be more tolerant to malic than to any other of the acids 
and to be inhibited most by acetic. At a pH value of 4.6 when malic acid was 
employed, Bacterium tabacum made a slight growth, whereas with acetic 
acid, the limit lies between 5.8-6.0. In the case of Bacillus carotovorus, inhibi¬ 
tion by these two acids occupies a much more restricted range since growth 
was checked between 4.6 and 4.8 by malic acid and 5.3 is the corresponding 
limit with acetic. Bacterium campestre is considerably more intolerant to 
formic acid than is Bacillus carotovorus since 5.5 and 4.8 represent the re¬ 
spective limits. 
Differences in vigor of growth on the agar slants are indicated by the same 
signs and the data are summarized in Table 2. 
The most striking fact exhibited by growth on solid media as contrasted 
with liquid media is the ability of the several organisms to withstand a greater 
H-ion concentration when grown on the former. In general, this difference 
appears to vary from 0.2-0.4 pH. Just as in liquid media, all organisms are 
less tolerant to acetic than to the other acids. Bacillus carotovorus appears to 
be considerably less sensitive to acetic acid, however, than any of the other 
organisms since its limit lies between pH 4.8-5.0 and the others between pH 
5.5-S.8. 
DISCUSSION 
The fact that acids of different hydrogen ion concentration can exert a simi¬ 
lar influence in checking bacterial growth has previously been noted, as has 
been indicated. Winslow and Lochridge (2), using B. coli and B. typhosus, 
found that the disinfecting power of acids was proportional to the concentra¬ 
tion of the hydrogen ion, but noted in the case of acetic acid an effect greater 
than that due to the H-ion concentration. In explanation, they suggest that 
this is due to the undissociated acid remaining in the solution. 
