CHAPTER IX 



THE ACTION OF CHEMICALS— CHEMOTAXIS AND 

 CHEMICAL DISINFECTION 



EXAMINATION of a drop of stagnant water under a medium-power 

 objective shows that the bacteria and infusoria it contains are not evenly 

 distributed through the liquid. They gather in clouds around particles 

 and flocculi of decaying organic matter, to which they seem to be attracted 

 as fish are attracted by bait, or ants by aphides. ' Instinctive ' is the term 

 (favoured of anthropomorphists) applied to such actions when performed by 

 the higher organisms, and yet we see them here carried out with unfailing 

 accuracy by creatures whose whole body consists of but a single cell. Are 

 then the bacteria too to be credited with instinct ? That would indeed be 

 a surprising conclusion ! 



This capability shown by unicellular organisms of being attracted by 

 food-stuffs was first carefully studied by Stahl (51). He showed that the 

 naked protoplasmic masses which form the plasmodia of the myxomycetes 

 were affected by a nutritive body placed in their neighbourhood. They turned 

 towards it on whichever side it was placed, suggesting by this action the 

 term Trophotropism as a suitable descriptive name for the phenomenon. At 

 about the same time Pfeffer (52) investigated from a more general chemical 

 standpoint the effects of such stimuli upon the movements of bacteria, 

 protozoa, and the spermatozoids of the higher cryptogams, and was able to 

 show that the nutritive value was not the sole and only determinant, but 

 that other more recondite factors were involved, factors based on the chemical 

 constitution of the substance employed as a stimulant. For this reason he 

 introduced the now generally adopted expression Chemotaxis. 



Pfeffer's method for the study of the chemotaxis of bacteria is simple 

 and efficient. A capillary tube, sealed at one end and about 5-10 mm. long, 

 is half filled with the liquid to be tested, e. g. a 5 per cent, slightly alkaline 

 solution of Liebig's Extract, or of peptone, in such a way that a bubble of 

 air remains in the closed end. The outer surface of the glass is carefully 

 cleaned from any traces of the broth, and it is placed in a drop of water 

 containing bacteria in such numbers as to be slightly turbid. In five or 

 ten seconds it can be seen that the bacteria are more thickly congregated 

 around the open end of the capillary than elsewhere, and in a few minutes 



