RELATION OF BACTERIA TO TEMPERATURE. 23 



B. proteus vulgaris, bacillus subtilis ; of obligatory anaerobes, 

 B. tetani, B. cedematis maligni, B. anthracis symptomatic! ; 

 while the great majority of pathogenic bacteria are faculta- 

 tive anaerobes. With regard to anaerobes, hydrogen and 

 nitrogen are indifferent gases. Many anaerobes, however, 

 do not flourish well in an atmosphere of carbon dioxide. 

 Very few experiments have been made to investigate the 

 action on bacteria of gas under pressure. A great pressure 

 of carbon dioxide is said to make the B. anthracis lose its 

 power of sporing, but it seems to have no effect on its 

 vitality nor on that of the B. typhosus. With the bacillus 

 pyocyaneus, however, it is said to destroy life. 



Temperature. For every species of bacterium there is 

 a temperature at which it grows best. This is called the 

 " optimum temperature." There is also in each case a 

 maximum temperature above which growth does not take 

 place, and a minimum temperature below which growth 

 does not take place. As a general rule the optimum tem- 

 perature is about the temperature of the natural habitat of 

 the organism. For organisms taking part in the ordinary 

 processes of putrefaction the temperature of warm summer 

 weather (20 to 24 C.) may be taken as the average 

 optimum, while for organisms normally inhabiting animal 

 tissues 35 to 39 C. is a fair average. The lowest 

 limit of ordinary growth is from 12 to 14 C., and the 

 upper is from 42 to 44 C. In exceptional cases growth 

 may take place as low as 5 C., and as high as 70 C. It 

 is to be noted that while growth does not take place below 

 or above a certain limit it by no means follows that death 

 takes place outside such limits. Organisms can stand 

 cooled below their minimum or heated beyond their maxi 

 without being killed. Their vital activity is merely paral 

 Especially is this true of the effect of cold on bacte 

 The results of different observers vary ; but if we take 

 an example the cholera vibrio, while Koch found that the 

 minimum temperature of growth was 16 C., a growth is 

 said to have been cooled to -32 C. without being killed. 

 With regard to the upper limit, few ordinary organisms in a 



