INFLUENCE OF TEMPERATURE 219 



After bacteria are once frozen, they do not seem to be affected by 

 any lower temperature. Macfadyen and Rowland found that they 

 tolerate very low temperatures remarkably well. Many bacteria 

 were not killed by a twenty hours' exposure to the temperature of 

 liquid hydrogen (—252°). Yeasts are not quite so resistant and the 

 mycelium of most molds is easily destroyed by freezing, while the spores 

 are hardier. 



Thermal Death-point. — Heating above the maximum tempera- 

 ture is quite harmful to bacteria, and^ the amount of injury increases 

 with the temperature. Recent experiments have shown that heat does 

 not kill bacteria instantaneously, but that we have an orderly process 

 as in the case of death by drying. This can be observed only in a 

 very narrow range of temperature, however, since the death rate rises 

 very rapidly with the increase of temperature. 10° increase may make 

 the death rate ten to one hundred times as great, and death' is almost 

 instantaneous. For most practical purposes, it is sufficient to state 

 the time and temperature neccessary to bring about complete sterili- 

 zation. It has become custoniary to define, as the thermal death- 

 point, the lowest temperature at which a culture will be killed in ten 

 minutes. As most bacteriologists will use very nearly the sametech- 

 nic, they will have fairly uniform numbers of cells to start with, 

 and therefore obtain fairly uniform results. 



The thermal death-point does not depend upon the species and 

 the temperature only. It varies with the age of the culture since 

 older cells are less resistant than younger ones especially if heated in 

 their own products. The medium in which the organisms are heated 

 is also of great significance. The fact that acid liquids, as fruit juices, 

 are more easily sterilized than neutral meat or vegetables is largely 

 due to a chemical (poisonous) action of the acids upon the bacteria. 

 But the greater resistance of tubercle bacteria in the sputum compared 

 with those suspended in salt solution cannot be so readily 

 accounted for. 



A necessary factor for the prompt destruction of organisms by 

 heat is the presence of moisture. The resistance of dry organisms 

 is remarkably higher than that of the same organisms in a liquid cul- 

 ture. The following table shows the death-point of yeast cells and 

 spores in a dry and moist state. 



