24 GENERAL MORPHOLOGY AND BIOLOGY. 



place, and a minimum temperature below which growth 

 does not take place. As a general rule the optimum tem- 

 perature 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. 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 

 1 6 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 tempera- 

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

 lose some of their properties when grown at unnatural 

 temperatures. Thus many pathogenic organisms lose their 

 virulence if grown above their optimum temperature, and 

 some chromogenic forms, most of which prefer rather low 

 temperatures, lose their capacity of producing pigment, e.g., 

 spirillum rubrum. 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. 



Effect of Light. Of recent years much attention has 

 been paid to this factor in the life of bacteria. Direct 

 sunlight is found to have a very inimical effect. One 

 observer found that an exposure of dry anthrax spores for 

 one and a half hours to sunlight killed them. When 



