CONDITIONS AFFECTING BACTERIAL MOTILITY 19 



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 

 temperatures. Thus many pathogenic organisms lose their 

 virulence if grown above their optimum temperature, and some 

 chromogenic forms, most of which prefer rather low tempera- 

 tures, 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 

 sunlight 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 ex- 

 cluded, though light plus heat is more fatal than light 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 exposure 

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

 light. Here, as with other factors, the results vary very much 

 with the species under observation, and a distinction must be 

 drawn between a mere cessation of growth and the condition of 

 actual death. Some bacteria especially occurring on the dead 

 bodies of fresh fish are phosphorescent. 



Conditions affecting the Movements of Bacteria. In some 

 cases differences are observed in the behaviour of motile bacteria, 

 contemporaneous with changes in their life history. Thus, in 

 the case of bacillus subtilis, movement ceases when sporulation 

 is about to take place. On the other hand, in the bacillus of 

 symptomatic anthrax, movement continues while sporulation is 

 progressing. Under ordinary circumstances motile bacteria 

 appear not to be constantly moving but occasionally to rest. 

 In every case the movements become more active if the 

 temperature be raised. Most interest, however, attaches to the 

 fact that bacilli may be attracted to certain substances and 

 repelled by others. Schenk, for instance, observed that motile" 

 bacteria were attracted to a warm point in a way which did not 



