CONDITIONS AFFECTING BACTERIAL MOTILITY 21 



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

 sunlight. 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 occur 

 when the bacteria were dead and therefore only subject to 

 physical conditions. Most important observations have been 

 made on the attraction and repulsion exercised on bacteria by 

 chemical agents, which have been denominated respectively 

 positive and negative chemiotaxis. PfefFer investigated this 

 subject in many lowly organisms, including bacterium termo 

 and spirillum undula. The method was to fill with the agent 

 a fine capillary tube, closed at one end, to introduce this into 

 a drop of fluid containing the bacteria under a cover-glass, and 

 to watch the effect through the microscope. The general result 

 was to indicate that motile bacteria may be either attracted or 

 repelled by the fluid in the tube. The effect of a given fluid 

 differs in different organisms, and a fluid chemiotactic for one 

 organism may not act on another. Degree of concentration is 

 important, but the nature of the fluid is more so. Of inorganic 

 bodies salts of potassium are the most powerfully attracting 

 bodies, and in comparing organic bodies the important factor 

 is the molecular constitution. These observations have been 

 confirmed by Ali-Cohen, who found that while the vibrio of 

 cholera and the typhoid bacillus were scarcely attracted by 

 chloride of potassium, they were powerfully influenced by potato 

 juice. Further, the filtered products of the growth of many 

 bacteria have been found to have powerful chemiotactic pro- 

 perties. It is evident that all these observations have a most 

 important bearing on the action of bacteria, though we do not 



