CHAPTER V. 



1. MOTILITY IN RESPONSE TO EXTERNAL STIMULI. 



WE have seen that bacteria are motile, and we have now to consider 

 in what manner external factors affect the power of movement. The 

 most important is the effect which is known as Chemiotaxis, the name 

 given to the power of attraction or repulsion which certain chemical 

 substances possess, for motile organisms. The best known example 

 is the attraction of oxygen for motile aerobic bacteria. If an aerobic 

 bacillus be placed in a drop of water and a glass coverslip be placed 

 over the latter, the individuals will swim towards the edges, being 

 attracted by the oxygen of the atmosphere. Conversely, an anaerobic 

 bacillus will be repelled by the oxygen of the atmosphere, and, under 

 the same circumstances, will move towards the centre of the coverslip 

 because this point is furthest removed from the oxygen. In these 

 cases the reason for the respective movements is obvious, for the well- 

 being of these bacteria depends in the one case on the presence of 

 oxygen, in the other on its absence. But attraction or repulsion is not 

 always guided by this consideration, for corrosive sublimate, the most 

 powerful germicide we know of, attracts bacteria, whilst most of the 

 substances which are used for the nutrition of bacteria are quite neutral 

 in this respect. 



Pfeffer has given us a good method of ascertaining the chemiotactic 

 power of any particular substance. A small portion of a fine capillary 

 tube, closed at one end, is first filled with the liquid to be examined, 

 and then placed in a drop of liquid containing some motile microbe. 

 The fineness of the bore prevents the liquid from running out of the 

 tube. If this liquid has an attraction for the bacteria, it will be found 

 that there is a concentration of the latter round the entrance to the 

 tube. On the other hand, if the liquid be neutral, no change in the 

 distribution of the bacteria will be observed, whereas if it exerts 



