124 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



pressure of six thousand atmosplieres continued for four- 

 teen hours was found to destroy all non-spore-forming 

 organisms. The spores of bacteria are not uniformly and 

 regularly killed even at pressures of twelve thousand atmos- 

 pheres. The pressures required to kill bacteria are so 

 enormous that they have not been found practicable in 

 the sterilization of food materials. In general bacteria that 

 are killed by high pressure are found to be difficult to 

 stain, only shadows being discernible. Gram-positive bac- 

 teria in general lose their specific staining reaction. 



Effect of Osmotic Pressure 



It will be recalled that the protoplasm of all cells is 

 bounded by a semipermeable membrane termed the ecto- 

 plast. Some substances in solution can pass through this, 

 others cannot. Water usually penetrates it readily. 

 Whenever the concentration of solutes on the interior is such 

 that osmotic pressure is greater on the inside of the cell 

 than on the outside, water passes in and the cell becomes 

 turgid. This is the normal condition of cells. When a 

 cell is plunged into a concentrated solution of some sub- 

 stance which will not pass through ectoplast, the water 

 passes from the inside of the cell to the exterior, the pro- 

 toplasm tends to shrink, and the cell is said to have under- 

 gone plasmolysis. On the other hand, when an organism 

 has become accustomed to growing in concentrated solutions, 

 such as syrups, and is then dropped into distilled water 

 the pressure on the interior of the cell is much greater than 

 on the exterior. The cell will then tend to swell and per- 

 haps burst. This condition is termed plasmoptysis. This 

 latter phenomenon may be readily observed by placing a 

 drop of distilled water over the tips of the hyphae at the 

 margin of a mold colony growing upon beer-wort agar. 

 The distilled water suddenly diminishes this concentration 

 locally, the pressure on the interior of the cell being such 



