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 temperature fatal to 

 the bacteria 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 or- 

 dinary determinations it is well to make a transfer at 40 C., 

 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 observed that all the 

 cultures grow except those heated beyond a certain point, say 

 62 C., when it can properly be concluded that 62 C. is the 

 thermal death-point. If all the transplantations grow, of 

 course the maximum temperature was not reached, and the 

 experiment 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 temperature as 60 C. 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 re- 

 moved as soon as it is heated through, another in five min- 

 utes, 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 and 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 



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