18 GENERAL MORPHOLOGY AND BIOLOGY 



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. Examples 

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

 obligatory anaerobes, b. tetani, b. oedematis 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. 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 tempera- 

 ture 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 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. 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 a certain limit it by 

 no means follows that death takes place outside such limits. 

 Organisms can resist cooling below their minimum or heating 

 beyond their maximum without being killed. Their vital activity 

 is merely paralysed. Especially is this true of the effect of cold 

 on bacteria. The results of different observers vary ; but if we 

 take as an example the cholera vibrio, Koch found that while 

 the minimum temperature of growth was 16 C., a culture might 



