122 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



temperature which will cause slow coagulaticn or cooking 

 of egg white. 



Growth Temperature Range. — Some bacteria have a very 

 wide growth temperature range ; for example, the Bacterium 

 coli will grow slowly at both 12° and 45° C, giving a growth 

 temperature range of at least 33°. Other bacteria, such as 

 some of the pathogenic forms, may have maximum temper- 

 ature at 40° and minimum temperature at 35°, giving a 

 growth temperature range of only 5°. It is apparent that 

 for such bacteria the temperature at which they are grown 

 must be carefully controlled if successful results are to be 

 secured. 



Thermal Death Point. — While the term thermal death 

 point is commonly used in literature, it is not entirely satis- 

 factory to express the particular idea. A thermal death 

 point is sometimes defined as that temperature which in a 

 given length of time will kill a particular organism. We 

 have already seen that all the organisms in a culture do not 

 die instantaneously upon being subjected to unfavorable 

 conditions, but that under a definite set of conditions, there 

 will be a definite rate of death. Theoretically it would be 

 better to designate the rate of death under certain standard 

 conditions at a definite temperature. The rate at which 

 bacteria die off at a given temperature is influenced by 

 several factors. Bacteria which produce spores are fre- 

 quently said to have two thermal death points, one for the 

 vegetative rod and another for the spores. The latter are 

 much more difficult to kill, that is, they die off less rapidly 

 under unfavorable conditions than do the vegetative cells. 

 The hydrogen ion concentration of the medium also is 

 important. The higher the hydrogen ion concentration or 

 the higher the hydroxyl ion concentration, the more rapidly 

 will bacteria die off at a given temperature. The presence 

 or alsense of moisture may also be important. Bacteria 

 that are not killed by drying are much more resistant in the 



