EFFECT OF EXTERNAL CONDITIONS ON BACTERIA 163 



basis. It may possibly be due to the production of certain 

 chemical substances in the tissues, as ozone or hypochlorous 

 acid, or by a stimulation of the cells to renewed activity, or 

 an increased phagocytosis. Some experiments have seemed 

 to indicate that prolonged exposure of cultures to the X-rays 

 may slightly diminish the vitality and virulence of the 

 germs. 



Movement. Bacteria apparently grow better when they 

 are in a state of rest. Experiments show that slight move- 

 ments are not important in affecting the growth of bacteria, 

 but that more violent movement, such as-, for instance, might 

 be obtained by attaching the culture to the piston of a rapidly 

 moving steam engine or a shaking machine, does have an in- 

 hibitory action. A practical phase of this subject manifests 

 itself in a discussion of the question of the effect of the move- 

 ment of a stream on the contained bacteria. It appears from 

 our present knowledge that such movement could not be much 

 of a factor in the purification of a stream. 



Pressure. Ordinary variations in pressure do not affect 

 either the life or the virulence of bacteria, and there is some 

 question whether extremely high pressures affect the cell. 

 Russell, in his work at Naples, found the same species of bac- 

 teria both at the surface and in deep-sea dredgings. Others, 

 on the other hand, say that 600 to 700 atmospheres of pressure 

 have an inhibitory effect on putrefactive processes. Roger 

 claims that a pressure of 2000 atmospheres interferes with the 

 virulence of the Bact. anthracis. 



