CHAPTER XVI 



THE DETERMINATION OF THE THERMAL DEATH-POINT 



OF BACTERIA 



SEVERAL methods may be employed for this purpose. Roughly, 

 it may be done by keeping a bouillon culture of the micro-organism 

 to be investigated in a water-bath whose temperature is gradually 

 increased, transplantations being made from time to time until the 

 fatal temperature is reached. 



It is economy to make the transplantations less frequently at 

 first than later in the experiment, when the ascending temperature 

 approaches a height dangerous to life. In ordinary determinations 

 it is well to make a transfer at 4oC., another at 45, another at 50, 

 still another at 55, and then, beginning at 60, make one for every 

 additional degree. The day following the experiment it will be ob- 

 served that all the cultures grow except those heated beyond a 

 certain point, say 62C., when it can properly be concluded that 

 62C. is the thermal death-point. If all the transplantations grow, 

 of course the maximum temperature was not reached, and the ex- 

 periment must be repeated and the bacteria exposed to still higher 

 temperatures. 



When more accurate information is desired, and one wishes to 

 know how long the micro-organism can endure some such tempera- 

 ture as 6oC. without losing its vitality, a dozen or more bouillon- 

 tubes may be inoculated with the organism to be studied, and stood 

 in a water-bath kept at the temperature to be investigated. The 

 first can be removed as soon as it is heated through, another in five 

 minutes, another in ten minutes, or at whatever intervals the thought 

 and experience of the experimenter shall suggest, the subsequent 

 growth in each culture showing that the endurance of the organism 

 had not yet been exhausted. By using gelatin, pouring each 

 culture into a Petri dish, and subsequently counting the colonies, it 

 can be determined whether many or only a few of the organisms in a 

 culture possess the maximum resisting power. To determine the 

 percentage, it is necessary to know how many bacteria were present 

 in the tubes before exposure to the destructive temperature. 'Ap- 

 proximately the same number can be placed in each tube by adding 

 the same measured quantity of a fluid culture to each. 



In both of the procedures one must be careful that the temperature 

 of the fluid in the test-tube is identical with that of the water in the 

 bath. A sterile thermometer introduced into an uninoculated tube 



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