Studies on Physiology of Some Plant Pathogenic Bacteria 
23 
DISCUSSION 
It may be of interest in connection with the data which have been presented 
above to call attention to points of contact with the work of others. Pasteur 
(2) recognized that the expression “thermal death point” has no significance 
when considered apart from the agents or processes involved in pasteurization 
as attested by his statement that “It is easy to show that these differences in 
temperature which are required to secure organic liquids from ultimate change 
depend exclusively upon the state of the liquids, their nature and above all 
upon conditions which affect their neutrality whether towards acids or bases.” 
It is beside the point to present here any exhaustive review of the litera¬ 
ture from Pasteur’s time to the recent studies. Suffice it to say that acidity 
and alkalinity have, in consequence of more exact methods for their measure¬ 
ment, come to be better known as agents which influence the thermal relations 
of bacteria and molds. Clark (3) pointed out the influence of hydrogen ion 
concentration on the so-called thermal death point of B. coli in a paper pre¬ 
sented to the Society of American Bacteriologists, in 1914, which contained the 
statement that “in cellular destruction temperature is to be regarded as an 
accelerating condition and that among the active agents concerned, the con¬ 
centration of the hydrogen ions of a medium may be of great significance.” 
Later studies by Cohen and Clark (4) show that B. coli can survive a tem¬ 
perature of 45°C. for different lengths of time, depending upon the two fac¬ 
tors, pH concentration and kind of acid used. The most marked reduction in 
number of viable organisms was found by them to be at the acid extremes. 
This effect of reaction on cellular destruction is entirely in accord with that 
indicated by the several plant pathogenes at both the H and OH extremes. 
One would also expect as indicated by the results with B. coli and by the 
previous studies on tolerance to acids (1) that the same pH concentrations se¬ 
cured through the use of different acids would modify the thermal death points 
shown in Table 1 of these plant parasites. This specific effect of acids would, of 
course, be most evident as the limits of tolerance are approached and indi- 
cates the close interaction and- interrelation between nature of the acid, pH 
concentration and temperature. 
This relationship of reaction and thermal death point obtains also among 
spore bearing bacteria as concluded by Bigelow and Esty (5) and Weiss (6) 
who found that as the H and OH ion concentration is increased, the time re¬ 
quired for the complete destruction of the spores is decreased. The latter au¬ 
thor found in addition that sodium chlorid lowers the thermal resistance of B. 
botulinus and that the rate of lowering increases rapidly as the concentra¬ 
tion of this salt is increased. In the light of investigations by Paul, Birstein 
and Reuss (7) this lowering of thermal resistance through the addition 
of NaCl is correlated with increased toxicity of the acids resulting from the 
reaction between the salt and acids. 
SUMMARY 
A study has been made of the relation of reaction to the thermal death point 
of Bacillus carotovorus, Bacterium glycineum, B. sojae, B. campestre, B. angu- 
latum and B. tabacum. The organisms were exposed to the different tempera¬ 
tures in plain bouillon adjusted w T ith HC1 or NaOH. Variations of 4° to 6°C. 
are noted between pH 5.4 and pH 9.0, points within the H and OH ion limits 
of tolerance. 
