CHAPTER XL 

 EFFECT OF CHEMICALS ON BACTERIA. 



Chemotaxis. It has been repeatedly demonstrated that bacteria, 

 like other free-moving organisms, are apparently attracted by cer- 

 tain chemical substances in solution (positive chemotaxis), and 

 repelled by others (negative chemotaxis) . 



Pfeiffer, who was the first to study this phenomenon, developed a 

 very simple and efficient method of studying it with bacteria. A 

 capillary tube, sealed at one end and from 5 to 10 mm. long, is 

 filled with a 5 per cent, slightly alkaline solution of Liebig's beef 

 extract or of peptone. The outer surface of the glass is carefully 

 cleaned from any traces of the bouillon and is placed in a drop of 

 water containing bacteria. In a few seconds the bacteria are found 

 to thickly congregate around the open end of the capillary tube. 

 According to the view held by Jennings, the swarming of bacteria 

 around any point, where favorable nutrient conditions exist is not to 

 be looked upon as due to a definite attraction exerted upon the 

 bacterial cell, but as caused simply by the tendency to remain at 

 those points where the conditions are favorable. But this does not 

 seem to be the true explanation, for had the capillary tube been 

 filled with sugar, or glycerin, which are the best foodstuffs and richest 

 sources of energy, there would have been no such gathering of the 

 the bacteria at the end of the tube. Moreover, a solution of 0.019 

 per cent, potassium chlorid plus 0.01 per cent, mercuric chlorid 

 attracts bacteria by reason of the potassium which it contains, but 

 they rush into the tube only to meet their death from the mercury 

 salt. 



The explanation given by Loeb seems to be more reasonable: 

 "Theoretically, we may assume that if substances diffuse in air or 

 in water, the particles move in a straight line away from the center 

 of diffusion. If they strike an organism whose surface is affected 

 by the diffusing substance on one side only, the contractile proto- 

 plasm, or the muscles, turning the tip or the head of the whole 

 organism toward that side, are thrown into a different state of con- 

 traction from their antagonists. The consequence is a turning or 

 binding of the tip of the head until symmetrical points of the chem- 

 ically sensitive surface of the body are struck by the line of dif- 

 fusion (or the diffusing particles) at the same angle. As soon as 

 this occurs the contractile elements on both sides of the organ or 

 organisms are in an equal state of contraction, and the animal will 



