TEMPERATURE 19 



Moisture. The presence of water is necessary for the con- 

 tinued growth of all bacteria. The amount of drying which 

 bacteria in the vegetative stage will resist varies very much in 

 different species. Thus the cholera spirillum is killed by two or 

 three hours' drying, while the staphylococcus pyogenes aureus 

 will survive ten days' drying, and the bacillus diphtheriae still 

 more. In the case of spores the periods are much longer. 

 Anthrax spores will survive drying for several years, but here 

 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. In 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 can be anaerobes, and those which are preferably 

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

 differences are manifested to a slight degree, but all such 

 organisms are usually grouped as facultative anaerobes, i.e., pre- 

 ferably aerobic but capable of existing without oxygen. Ex- 

 amples of obligatory aerobes are b. proteus vulgaris, b. subtilis ; of 

 obligatory anaerobes, b. tetani, b. cedematis maligni, while the 

 great 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 or on that of the b. typhosus. In the case 

 of 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 temperature is 

 about the temperature of the natural habitat of the organism. 



