CONDITIONS AFFECTING BACTERIAL MOTILITY 21 



.bacteria, though it takes a much longer exposure 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 

 substances, salts of potassium are the most powerfully attracting 

 bodies, and in comparing organic bodies the important factor 

 is the molecular constitution. Further, the filtered products of 

 the growth of many bacteria have been found to have powerful 

 chemiotactic properties. It is evident that all these observa- 

 tions have a most important bearing on the action of bacteria, 

 though we do not yet know their true significance. Correspond- 

 ing chemiotactic phenomena are shown also by certain animal 

 cells, e.g., leucocytes, to which reference is made below. 



