THE BEHAVIOR OF BACTERIA 29 



of movement under certain conditions. If we place water containing 

 many Spirilla on a slide, allowing some small air bubbles to remain be- 

 neath the cover-glass, we find after a time that the bacteria are collecting 

 about the bubbles. The course of events in forming the collections is 

 seen to be as follows: At first the Spirilla are scattered uniformly, 

 swimming in all directions. They pass close to the air bubble without 

 change in the movements. But gradually the oxygen throughout the 

 preparation becomes used up, while from the air bubble oxygen diffuses 

 into the water. After a time therefore the bubble must be conceived as 

 surrounded by a zone of water impregnated with oxygen. Now the bac- 

 teria begin to collect about the bubble. They do not change their direc- 

 tion of movement and swim straight toward the center of diffusion of 

 the oxygen. On the contrary the movement continues in all directions 

 as before. A Spirillum swimming close to the bubble into the oxygen- 

 ated zone does not at first change its movement in the least. It swims 

 across the zone until it reaches the other side, where it would again 

 pass out into the water containing no oxygen. Here the reaction oc- 

 curs; the organism reverses its movement and swims in the opposite 

 direction. If the specimen has flagella at each end, it continues its re- 

 versed movement until the opposite side of the area containing the oxygen 

 is reached; then the movement is reversed again. This is continued, 

 the direction of movement being reversed as often as the organism 

 comes to the outer boundary of the zone of oxygen within which it is swim- 

 ming. 1 Thus the bacterium oscillates back and forth across the area of 

 oxygen. Specimens having flagella at but one end swim backward 

 only a short distance after reaching the boundary of the area, then start 

 forward again. 



As a result of this way of acting the bacterium of course remains in 

 the oxygenated area. The latter thus retains every bacterium that 

 enters it. Many bacteria, swimming at random, enter the area in the 

 way described, react at the outer boundary, and remain; thus in the 

 course of time the area of oxygen swarms with the organisms, while 

 the surrounding regions are almost free from them. The finding of the 

 oxygen then depends upon the usual movements of the bacteria, not 

 upon movements specially set in operation or directed by the oxygen. 



Thus the positive and negative reactions of the bacteria are pro- 

 duced in the same way; both take place through the reversal of the 

 movement when stimulated. The stimulus is some change in the na- 

 ture of the surrounding medium. In the negative reaction the change 

 is from ordinary water to water containing some chemical; in the posi- 



1 The bacterium may of course come against the bubble itself ; the movement is then 

 reversed in the same way. 



