78 BACTBEIOL.OGY. 



ation of the medium and the presence of germicidal sub- 

 stances can be easily demonstrated. Such a medium when' 

 subsequently inoculated with a fresh culture will remain 

 sterile. 



The effect of X-rays on bacteria has been the subject 

 of considerable study. These rays, as in the case of the 

 electric current, can be said to be without any direct action. 



Chemotaxis. 



The lower forms of life exhibit a singfular behavior with 

 reference to certain chemical substances. If, for instance, 

 a minute capillary tube is partly filled with a solution of 

 pepton or meat extract and the end of the tube is placed in 

 a drop of water which has been inoculated with a small 

 number of motile bacteria it will be found that, in a few 

 minutes, all the bacteria will have gathered around the 

 tube. This attraction of the bacteria by the chemical sub- 

 stance in the tube is known as chemotaxis. The various 

 chemical substances will show different degrees of attrac- 

 tion. Moreover, the concentration of the solution has 

 much to do with the result. Other compounds, such as 

 acids, alkalis and alcohol do not attract bacteria and, in- 

 deed, may be said to exert a repellant action. 11; is cus- 

 tomary, therefore, to speak of positive and of negative 

 chemotaxis. 



The phenomenon of chemotaxis, which was first ob- 

 served on higher plants, and then on lower forms, has been 

 utilized to explain the behavior of the cells of the body in 

 the presence of invading bacteria. It should be remem- 

 bered that there is no satisfactory explanation of the 

 phenomenon itself. 



