J. RUSSELL ESTY 



289 



Tanner and McCrca' also noted variations in the resistance of different strains under 

 controlled conditions. Spores in sealed tubes exhausted to 17 mm. were destroyed within 5 

 hours at 100° C, 2 hours at 105° C, i^ hours at iio°C., 40 minutes at 115° C, and 10 

 minutes at 120° C. A longer time was required for spores of the same age in open tubes than 

 in tubes exhausted and sealed. 



VARIATIONS IN THERMAL DEATH-TIMES 



From the foregoing statements, which are typical examples of all thermal death- 

 point studies, there appears to be considerable variation in the results obtained by 

 different investigators on the same group of organisms. The question arises, Why do 

 such differences exist and how can they be interpreted? Several explanations may be 

 offered to account for these discrepancies. 



Recent researches have shown that the accurate determination of the thermal 

 death-point of bacteria depends on the careful consideration of several very important 

 factors. It is not, as it was formerly considered, a simple procedure of subjecting the 



TABLE I 



micro-organisms, vegetative or spore, to heat and then removing a loopful or small 

 portion of the treated material to various sorts of media to determine whether or not 

 the organisms had been killed. Moreover, there is no one time and temperature com- 

 bination which alone may be defined as the thermal death-point, as the time varies 

 with the temperature at which the organisms are heated. There is, however, a definite 

 time-temperature relationship, the time decreasing as the temperature increases. In 

 view of this, the term "thermal death-time"^ probably more appropriately designates 

 this relation than "thermal death-point." 



It must be emphasized that the resistance of bacteria to moist heat is not a con- 

 stant but a variable influenced by certain conditions. From previous statements con- 

 cerning the spore resistance of CI. tetani and CI. boiidinmn, it is apparent that the 

 heat resistance of spores of different strains of the same organism fluctuates within 

 wide limits, even with identical conditions of cultivation, spore production, and heat 

 treatment. Similar variations have been noted with vegetative cells, and the following 

 examples may be cited. Based upon a study of 174 cultures of B. coll, Ayers and 

 Johnson^ found that all the cultures survived heating for 30 minutes at 51.7° C, 

 54.59 per cent at 60° C, 6.89 per cent at 62.8° C, 0.57 per cent at 65.6° C, and 



' Tanner, F. W., and McCrea, F. D.: /. Bad., 8, 269. 1923. 



2 Mentioned for the first time by Bigelow, W. D.: J. Infect. Dis., 29, 528. 1921, 



3 Ayers, S. H., and Johnson, W. T., Jr.: /. Agric. Research, 3, 401. 1915. 



