THE BEHAVIOR OF BACTERIA 



31 





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Fig. 27. — Collections of Spirilla, a, and a ciliate in- 

 fusorian Anophrys, 6, at the corner of the cover-glass, 

 and about a bubble. Each remains in a narrow zone 

 a certain distance from the air surface, the bacteria 

 farther away than the infusoria. After Massart. 



gen, certain differences are to be observed in different species. Spirilla 

 usually gather in a narrow zone a short distance from the air surface, 

 while Bacterium termo and most other species collect in another zone, 

 a little closer to the air. These relations are illustrated for Spirilla 

 and certain infusoria in Fig. 27. In such cases it is found that the 

 reversal of movement is 

 brought about in two differ- 

 ent regions. Passage from 

 the zone in which the quan- 

 tity of oxygen is adapted to 

 the particular species, to a 

 region having less oxygen, 

 causes the reversal; passage 

 to a region having more oxy- 

 gen (next to the air surface) 

 causes the reversal with even 

 greater precision. As a re- 

 sult, each species remains swimming about within the narrow zone 

 adapted to it, at a short distance from the air. 



Thus any given species is adapted to a certain concentration of oxy- 

 gen, which may be called its optimum. Passage from the optimum in 

 either direction — toward more oxygen or less oxygen — causes the 



reversal of movement, so that the 

 bacteria remain in the optimum. 



Oxygen is of course necessary, or 

 at least useful, to these bacteria ; most 

 of them become immobilized soon if 

 oxygen is excluded from the water. 

 The reversal of movement on passing 

 to a region of less oxygen is thus an 

 adaptive reaction. It is probable that 

 the concentration in which each spe- 

 cies tends to remain is that most 

 favorable to its life activities. Some 

 bacteria (the so-called anaerobic spe- 

 cies) do not require oxygen, and these 

 bacteria do not collect in an oxygen- 

 ated area. One of these, Amylobacter, is known to avoid oxygen in 

 all effective concentrations; that is, it reverses its movement on com- 

 ing to a region containing oxygen (Rothert, 1901). 



Many bacteria collect in various other chemicals in the same manner 

 as in solutions of oxygen (see Fig. 28). Such collections are usually 



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Fig. 28. — Collection of Chromatium 

 weissii in and about a capillary tube con- 

 taining 0.3 per cent ammonium nitrate. 

 After Miyoshi (1897). 



