CHEMOTAXIS 81 



emigration of leucocytes towards a bacterial focus (53). An exact knowledge 

 of the conditions involved is impossible, inasmuch as the composition of the 

 fluids in which the tissues are bathed and the amount of chemotactic 

 substances they contain are entirely unknown. For this reason considerable 

 discretion must be exercised in the use of the word chemotaxis if it is not to 

 become a mere shibboleth (see Ch. XVII). 



In the case of some substances, peptone for instance, the absolute 

 amount sufficient to cause perceptible reaction is very minute. Pfeffer 

 calculated that a capillary filled with o-oi .per cent, peptone solution, 

 a strength just sufficient to attract bacteria in water, contained only 

 2 *nit7ffT7F?n7 f a milligram of peptone. And yet compared with the size of 

 the bacterial cells this quantity is not disproportional. 



Like all other processes that depend ultimately upon the activity of the 

 living cell, the phenomenon of chemotaxis is extremely obscure. This 

 much however we know, that the effect of the substances diffusing from 

 the capillary is to cause the bacteria to turn (hence chemo-taxts) in such 

 a way that their longer axes are parallel to the direction of the diffusion 

 stream. They then swim either against the stream (positive chemotaxis) or 

 with it (negative chemotaxis). Why one substance should act positively 

 and another negatively is at present quite inexplicable. A more detailed 

 discussion would be beyond the scope of this treatise, and would be at best 

 fragmentary. Those substances which in weak solutions exercise a positive 

 action on bacteria have in some cases (KC1 19 per cent.) the same effect 

 when the solution is concentrated ; in other cases the bacteria are repelled. 

 Neutral salts such as KC1 and NaCl can be endured in very high concen- 

 trations. The hay bacillus, for instance, grows well in an infusion containing 

 9 per cent. NaCl, 5 per cent, sal ammoniac, n per cent. KC1, or 10 per 

 cent. KNO 3 . Such substances are not poisonous, and arrest growth finally 

 by the osmotic pressure they set up. 



Of great interest are those poisonous compounds which even in minute 

 quantities destroy the life of the cells. They are not by any means 

 especially poisonous for bacteria. A one-tenth per cent, solution of corrosive 

 sublimate (HgCl 2 ) kills tubercle bacteria in ten minutes, and the cells of 

 algae are destroyed just as soon, if not sooner. In a I per cent, solution 

 of carbolic acid both tubercle bacilli and ordinary plant cells are killed in 

 one minute. With few variations and exceptions the protoplasm of all 

 organisms is destroyed in about the same time by the more powerful poisons. 



The destruction of bacteria by poisons chemical disinfection (54) is 

 employed in all cases where the application of the methods described in 

 the last chapter is impossible on account of the injury caused by the heat. 



The resistance of bacteria to chemicals is not only different in different 

 species, but varies also according to many external circumstances. It is 

 greatest when the organisms are growing under the most favourable 



