CHAPTER VI. 

 INFLUENCE OF MECHANICAL EFFECTS. 



PRESSURE. The resistance of microorganisms to mechanical pressures 

 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. pseudodiphtherioe, M. 

 pyogenes var. aureus, Oidium lactis and Saccharomyces cerevisice survived, 

 while seven other organisms lost the power of multiplication. Some of 

 these were not dead, however, since they retained their motility for several 

 days. It is noteworthy that high pressure will destroy one quality 

 (multiplication) and not affect another (motility). Pigment-production 

 and virulence of pathogenic bacteria were either diminished or lost com- 

 pletely. 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. Microorganisms 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 coefficient 

 of compression. 



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

 pressure directly, but to compressed gas. 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 antiseptic; it depends 

 upon the chemical qualities of the gas, upon the pressure (concentration) , 

 upon the temperature, and upon the organism. 



Some data have been given already in the chapter on oxygen require- 



* One atmosphere is i kg. pressure per square centimeter (or about 15 pounds per square 

 inch). 



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