20 GENERAL MORPHOLOGY AND BIOLOGY. 



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. Espe- 

 cially 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. with- 

 out 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 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. 



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 aTnd a half hours 

 to sunlight killed them. When they were moist, a much longer 

 exposure was necessary. Typhoid bacilli were 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 ex- 

 cluded, though light plus heat is more fatal than hght alone. 

 In direct sunlight it is chiefly the green, violet, and, it may be, 

 the ultra-violet rays which are fatal. Diffuse daylight has also 

 a bad effect upon bacteria, though it takes a much longer ex- 

 posure to do serious harm. A powerful electric light is as fatal 

 as sunlight, but the so-called X-rays are quite without any 



