58 BACTERIOLOGY 



similar destructive dissociation within the bacteria them- 

 selves may result from the action of the current. 



The evidence at hand 'does not permit of the acceptance 

 of either of these suggestions as the correct interpretation of 

 the results. 



Chemotaxis. Another interesting biological peculiarity of 

 bacteria is that discovered by Engelmann and by Pfeffer, 

 known as chemotaxis. This term applies to the peculiar 

 phenomena of attraction and of repulsion that are exhibited 

 by motile bacteria when in the presence of solutions of bodies 

 of various chemical composition. It was demonstrated 

 that the bacteria in decomposing infusions accumulate in 

 great numbers in the neighborhood of the sources of oxygen. 

 In a hanging-drop of such an infusion the bacteria will be 

 seen to accumulate in a dense mass along the margin or 

 around the edge of small bubbles of air in the fluid. Even 

 plant cells in the infusion, whose chlorophyl sets free oxygen 

 in the light, are surrounded by large numbers of bacteria. 

 The positive chemotactic affinity between oxygen and 

 bacteria was employed by Engelmann as a basis for the 

 demonstration of small quantities of oxygen in studying the 

 influence of various kinds of light upon the assimilation of 

 green plant-cell. Pfeffer showed that when a neutral fluid 

 (a drop of water) containing motile bacteria is brought in 

 contact with a weak solution of either peptone, sodium 

 chloride, or dextrin, the bacteria are at once attracted 

 toward the solution; this reaction is designated "positive 

 chemotaxis." On the other hand, if brought in contact with 

 an acid, an alkaline, or an alcoholic solution, the bacteria 

 are repelled or driven from the point at which the two fluids 

 are diffusing; that is, they exhibit "negative chemotactic" 

 affinities. The significance of these reactions is not under- 



