CHAPTER V 

 INFLUENCE OF MECHANICAL EFFECTS 



Pressure. — The resistance of microorganisms to mechanical pres- 

 sures is very great. Pressures of 3,000 atmospheres* will not kill the 

 majority of bacteria in four hours. They are, however, weakened and 

 some species will die. A specific difference between the molds, yeasts, 

 and bacteria in this particular does not seem to exist. Of the organisms 

 exposed to 2,000 atmospheres for ninety-six hours. Bad. anthracis, Bad. 

 pseudodiphtkerice, M. pyogenes var. aureus, Oidium ladis and Saccharo- 

 myces cerevisia survived, while seven other organisms lost the power of 

 multiplication. Some of these were not dead, however, since they 

 retained their motility fof several days. It is noteworthy that high 

 pressure will destroy one quality (multiplication) and not effect another 

 (motility). Pigment-production and virulence of pathogenic bacteria 

 were either diminished or lost completely. The resistance against 

 high pressure is necessary for the organisms which cause the decay 

 of organic matter at the bottom of the oceans. Vertebrates breathe 

 oxygen in the form of gas or have at least an organ filled with gas (fish 

 bladder) ; the volume of gas is changed considerably by slight changes 

 of pressure; this will affect organisms depending on gas. Microorgan- 

 isms do not require gas as such. They can absorb gases only in 

 solution. A change of pressure therefore will not cause a change of 

 volume, since liquids have a very small coefficieiit of compression. 



The situation is entirely different if the liquid is not exposed to the 

 pressure directly, but to compressed air. In this case, the chemical 

 effect of the gas is the deciding agent. The higher the pressure, the 

 more gas will be dissolved in the culture medium. The fatal pressure 

 under these conditions will vary as much as the fatal dose of an antisep- 

 tic; it depends upon the chemical quaUties of the gas, upon the pressure 

 (concentration), upon the temperature, and upon the organism. 



•One atmosphere is i kg. pressure per square centimeter (or about is pounds per square 



inch). 



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