48 BACTERIOLOGY. 



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 understood, 

 but it has been aptly suggested that they may be funda- 

 mentally analogous to the specific positive and negative 

 affinities exhibited by the ions (see page 91) resulting 

 from the dissociation of electrolytes, and that they may 

 " have their explanation in the forces of ionic attraction 

 and repulsion." 1 In this connection it is important to 

 note that the wandering cells of the animal body, the 

 leucocytes, exhibit also these chemotactic phenomena; 

 and it is especially necessary to a complete comprehen- 

 sion of the process of suppuration to bear in mind that 

 among the substances which have the greatest attraction 

 for these wandering cells, are the products of growth of 

 certain bacteria in some cases, and the protoplasmic con- 

 stituents of the bacteria themselves in others. 



To summarize briefly the foregoing it may be said, in 

 general, that for the growth and development of bacteria 

 organic matter of a neutral or slightly alkaline reaction, 

 in the presence of moisture and at a suitable temperature, 

 is all that is necessary. From this can be formed some idea 

 of the omnipresence in nature of these minute vegetables. 

 Bacteria may be found wherever these conditions obtain. 



1 Bead Sewall on "Some Eelations of Osmosis and Ionic Action in 

 Clinical Medicine," International Clinics, vol. xi., Eleventh Series. 



