BACTERIOLOGY. 55 



Moisture is essential to the growth of 

 Moisture, bacteria, in fact, complete drying, except 



in the case of spores, is followed by death. 

 Thus the spirilla of Asiatic cholera are killed in a few 

 minutes by drying. However, too much reliance can- 

 not be placed on this factor, since an infinitesimal 

 amount of moisture suffices to keep them alive indefi- 

 nitely. 



The most favorable temperature for the 

 Tempera- gj-Q^^-]^ ^f pathogenic bacteria is that of 

 ''''^^' our bodies, 98.6° F. (37° C). They will 

 also readily multiply at lower temperatures if other 

 circumstances permit. Thus the temperature of the 

 ordinary summer day (70° F., 24° C.) is sufficient for 

 rather rapid multiplication. Below this temperature, 

 however, growth becomes slower as the temperature is 

 reduced, and practically ceases at 16° C. (60.8° F.). 

 Above the temperature at which bacteria grow best 

 (optimum temperature), only a few degrees suffice to 

 prevent growth (43° C, 109.4° F.). 



Oxygen is another essential to the growth 

 Oxygen, of bacteria, but there is considerable 



difference in the manner in which the 

 various species obtain it. The majority grow best when 

 oxygen, as found in the air, is supplied. A majority, 

 however, will not grow in the presence of free oxygen, 

 being compelled to obtain their oxygen from material 

 in which it is in combination with other elements. 



