RELATION TO GASEOUS ENVIRONMENT 19 



quite dry. When organisms have been subjected to such hostile 

 influences, even though they survive, it by no means follows that 

 they retain all their vital properties. 



Relation to Gaseous Environment. — The relation of bacteria 

 to the oxygen of the air is such an important factor in the life 

 of bacteria that it enables a biological division to be made among 

 them. Some bacteria will only live and grow when free oxygen 

 is present. To these the title of obligatory aerobes is given. Other 

 bacteria will only grow when no free oxygen is present. These are 

 called obligatory anaerobes. In still other bacteria the presence 

 or absence of oxygen is a matter of indifference ; such organisms 

 are usually denominated facultative anaerobes, — they being pre- 

 ferably aerobic but capable of existing without oxygen. An 

 example of an obligatory aerobe is b. subtilis ; of an obligatory 

 anaerobe, b. tetani, while the great majority of pathogenic 

 bacteria are facultative anaerobes. The precise part played by 

 oxygen tension in the growth of anaerobes may require further 

 investigation, as, in certain species, anaeroblosis is a relative 

 property. 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 on 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 seems to have no effect on its 

 vitality or on that of the b. typhosus ; in the case of the bacillus 

 pyocyaneus, however, such pressure 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 temperature is 

 about the temperature of the natural habitat of the organism. 

 For organisms taking part in the ordinary processes of putrefac- 

 tion 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° O, and the upper is from 42° to 44° C. In exceptional 

 cases growth may take place as low as 5° O, and as high as 

 70° C. Some organisms which grow best at a temperature of from 

 60° to 70° C. have been isolated from dung, the intestinal tract, 

 etc. These have been called thermophilic bacteria. It is to 

 be noted that while growth does not take place below or above 



