THE BEHAVIOR OF BACTERIA 



3 1 



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. 

 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 



In such cases it is found that the 



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



o-pn (next to the air surfaced fusorian Anophrys, b, at the corner of the cover-glass, 

 gen (nexi to tne dir bUridLC; and about a bubble Each rema ins in a narrow zone 



Causes the reversal with even a certain distance from the air surface, the bacteria 



a farther away than the infusoria. After Massart. 



greater precision. As a re- ' 



suit, 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 



'^M^- '-•'•'•'•.'•' to a re gi° n °f l ess ox yg en is thus an 



•:£§||p ; :'.'• ••'• .'■' •'• adaptive reaction. It is probable that 



••..;';. ^ r ' •■'"' •'■. ;'•;-.'••■ .' the concentration in which each spe- 



'■'. ■■:■:'■■'■ : V V: •*•' ' ' '' cies tends to remain is that most 



favorable to its life activities. Some 



Fig. 28. — Collection of Chromatium ... 111 j. • 



wdssii in and about a capillary tube con- bacteria (the so-called anaerobic spe- 



taining 0.3 per cent ammonium nitrate. c J es ) ^ Q no t require OXVgen, and these 

 After Miyoshi (1897). ' . ? 11 /• 



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, iqoi). 



Many bacteria collect in various other chemicals in the same manner 

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



