20 GENERAL MORPHOLOGY AND BIOLOGY 



For organisms taking part in the ordinary processes of putrefac- 

 tion 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 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 

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

 lose some of their properties when grown at unnatural tempera- 

 tures. 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. 



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. It has been found that an 

 exposure of dry anthrax spores for one and a half hours to sun- 

 light kills them. When they are moist, a much longer exposure 

 is necessary. Typhoid bacilli are killed in about one and a half 

 hours, and similar results have been obtained with many other 

 organisms. In such experiments the thickness of the medium 

 surrounding the growth is an important point. Death takes 

 place more readily if the medium is scanty or if the organisms 

 are suspended in water. Any fallacy which might arise from 

 the effect of the heat rays of the sun has been excluded, though 

 light plus heat is more fatal than light alone. In direct sunlight 

 it is chiefly the green, violet, and the ultra-violet rays which are 

 fatal. The last-mentioned rays, however produced, have a 

 powerful bactericidal action. Diffuse daylight has also a bad 

 effect upon bacteria, though it takes a much longer exposure 



