TEMPERATURE AND EFFECT OF LIGHT 19 



gases. Many anaerobes, however, do not flourish well in an 

 srttnosphere 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. 

 For organisms taking part in the ordinary processes of putrefac- 

 tion the temperature of warm summer weather (20 to 24 C.) 

 may l>u 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 

 > 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 thermophUic bacteria. It is to 

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

 ;i certain limit, it by no means follows that death takes place 

 Hitside 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 olisrrvers 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 be cooled to -32 C. 

 without being killed. With regard to the upper limit, few 

 ordinary organisms in a spore-free condition will survive a 

 temjKirature of 57 C., if long enough applied. Many organisms 

 lose some of their properties when grown at unnatural temper- 

 atures. Thus many pathogenic organisms lose their virulence 

 it grown aliovc their optimum temperature, and some chromogenic 

 tonns must <>!' \\hich prefer rather low temperatures, lose their 

 capacity of producing pigment, </.//. spirillum rabmnL 



Effect of Light. Of recent years much attention has been 

 paid to this factor in the life of bacteria. Direct sunlight is 



