52 AGRICULTURAL BACTERIOLOGY 



growth, a point is soon reached at which injury to the cells 

 takes place. The weaker cells are first destroyed ; the more 

 resistant only as the heat is applied for a longer period 

 or a higher temperature is used. The point at which all 

 of the cells of a given culture are destroyed is known as the 

 thermal death point. This temperature will depend on the 

 length of exposure to heat, and on the amount of moisture 

 present. The smaller the percentage of water, the less in- 

 jurious will be the effect of the heat. Heat injures the 

 cell through coagulation of the protoplasm. The tempera- 

 ture required to cause coagulation rises rapidly as the water 

 content of the cells decreases. This fact is the reason for 

 the greater effectiveness of moist heat as compared -with dry 

 heat in sterilizing processes. 



The chemical reaction of the liquid is also a factor in de- 

 termining the destructive action of heat on microorganisms. 

 In an acid-reacting liquid they are .more easily destroyed 

 than in a neutral solution. Bacterial spores, as has been 

 stated, are extremely resistant to heat. 



Relation to oxygen. Oxygen is an essential element for 

 the growth of all life. The higher forms of life, both plant 

 and animal, are adjusted to rather definite percentages of 

 oxygen in the air. This is not true of the microorganisms 

 as a class. Some can grow in the total absence of free oxy- 

 gen; still others can grow in percentages far above that of 

 the atmospheric air. 



The term anaerobic is applied to those that can grow in 

 the absence of free oxygen. They can also grow in the pres- 

 ence of reduced amounts of free oxygen. When they grow 

 in the absence of free oxygen, they must draw their oxygen 

 supply from combined sources, as from the sugars, nitrates, 

 or sulphates. In this mode of securing oxygen, they are 

 not unlike the unit cells of the animal body that derive their 



