TEMPERATURE. 19 



again moisture enables them to resist longer than when they are 

 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 oxygen is 

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

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

 called obligatory anaerobes. To still other bacteria the presence 

 or absence of oxygen is a matter of indifference. This group 

 might theoretically be divided into those which are preferably 

 aerobes, but could be anaerobes, and those which are preferably 

 anaerobes, but could be aerobes. As a matter of fact, such 

 differences are manifested to a slight degree, but all such organ- 

 isms are usually grouped as facultative anaerobes, i.e. prefer- 

 ably aerobic but capable of existing without oxygen. Examples 

 of obligatory aerobes are B. proteus vulgaris, B. sub tills ; of 

 obligatory anaerobes, B. tetani, B. oedematis maligni, while- the 

 gjeat majority of pathogenic bacteria are facultative 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, 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 " opti- 

 mum 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 temper- 

 ature of the natural habitat of the organism. For organisms 

 taking part in the ordinary processes of putrefaction the temper- 

 ature 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 



