IV. Thermal Death Points of Some Bacterial Plant Pathogenes 
in Relation to Reaction of the Medium 
Those who have interested themselves in a study of the reaction of culture 
media know of its profound influence upon the morphology of bacteria, their 
viability, enzyme formation, pigment production, and rate of fermentation and 
putrefaction as recorded in scores of instances. Quite a few investigations, 
too, but all of which so far as is known, have been conducted outside the field 
of plant pathology, have noted that hydrogen ion concentration affects also 
( 
the thermal death point. It is the present purpose, therefore, to report on 
studies of several organisms pathogenic to plants in order to show the influence 
of this factor upon their thermal death points. 
METHODS 
Subcultures of the organisms employed in the previous study (1) were used 
in these tests. Bacillus carotovorus, Bacterium campestre and B. glycineum. 
the latter from type material, were obtained through the courtesy of the De¬ 
partment of Plant Pathology of the University of Wisconsin. The others, Bac¬ 
terium tabacum, B. sojae and B. angulatum were isolated in this laboratory, 
the first two of which are subcultures from type strains. Cultures of these 
several organisms were grown for twenty-four hours in a medium consisting 
of 2 per cent agar, made up with a watery extract from boiled potatoes, plus 
2 per cent dextrose. Transfers were made from these vigorously growing cul¬ 
tures to bouillon prepared from 1 per cent Armour’s peptone, 0.3 per cent Lie¬ 
big’s beef extract and 0.5 per cent NaCl in distilled water. All of the bouillon 
was made in the same lot, was then divided into ten portions, flashed, cooled, 
adjusted to the desired pH concentration with HC1 or NaOH, tubed, and then 
incubated for forty-eight hours to determine its sterility. Triplicate sets of 
each organism in each of the ten concentrations were exposed for ten minutes 
to the temperature conditions of the water bath. This bath had a capacity of 
about eight gallons, and could, with vigorous stirring, be maintained within a 
range of two-tenths degree of the desired temperature. All exposures were 
made as soon as possible after the transfers had been made. Further, all 
were incubated for four days at 20-25°C. after exposure and death was deter¬ 
mined by their inability to cloud the media. 
RESULTS 
The summary in Table 1 shows the differences obtained. In the case of vis¬ 
ible clouding of the media the sign -f- is employed and no visible growth is in¬ 
dicated by —. 
Under the conditions of this experiment as modified by the effect of reaction, 
the thermal death point of B. sojae lies between 44-48°C. of B. glycineum, 43- 
48°C. of B campestre 45-51°C. of B. angulatum 45-51°C. of B. tabacum 
46-51°C'. and of B. carotovorus 47-51°C. In general it will be seen that the 
greatest sensitivity to higher temperatures appears to be manifested at ex¬ 
treme reactions. 
