Effects of Physical Forces on Bacteria 165 



tops," bacteria under gas pressure may expand to the point of 

 disintegration. Some bacteria found at great depths in the sea, 

 however, show remarkable powers of adjustment to changes in air 

 pressure. Gram negative organisms are much more susceptible 

 to sudden release of carbon dioxide pressure than the tougher 

 gram positive species. The latter organisms may be killed in the 

 process but visible disruption of the cells is not evident. Fish have 

 been recovered from the ocean depths where the pressure may 

 reach 300 atmospheres. A unique pressure bladder allows these 

 fish to adjust to pressure changes within limits. 



If oxygen is the gas employed, a compression of 100 atmospheres 

 above bacterial cultures may increase the amount of dissolved 

 oxygen in the cells by as much as one hundred times the normal 

 values. Many cells "suffocate" from so much oxygen, a toxic effect 

 comparable to chemical sterilization. Inert gases, such as hydrogen 

 and nitrogen, do not have this same effect, but they may alter 

 staining characteristics of the organisms. Acid-fast bacteria may 

 lose their acid-fastness, and gram positive bacteria may lose their 

 gram positiveness. Carbon dioxide, however, may aid in the 

 death of cells exposed to increased pressure of this gas. The 

 amount of carbon dioxide in carbonated drinks is no small factor 

 in holding down the bacterial count in these products. Many of 

 these popular drinks have a very low pH which is unfavorable 

 for bacteria, and it may be harmful to the enamel on the teeth of 

 persons who drink these carbonated beverages to excess. Many 

 fish and meats that would ordinarily undergo slow decomposition 

 as the result of action by bacterial psychrophiles at or near 0° C, 

 can be preserved for extended periods when stored in a carbon 

 dioxide atmosphere. A high oxygen pressure (8 to 10 atmos- 

 pheres), when coupled with low temperatures, has been success- 

 fully employed on an experimental basis for the preservation of 

 milk in the Hofius Process. 



OSMOTIC PRESSURE 

 Osmotic Pressure is defined as the diffusion pressvire of a solvent 

 passing through a semi-permeable membrane. This is an equaliz- 



