KIND OF FOOD REQUIRED 67 



when in solution, while in solution it reacts and after it has served 

 its purpose the waste products are carried from the cell in solution. 



Osmotic Pressure. If a cell be placed in a strong salt solution, 

 there is a shrinking of the cell which may result in plasmolysis. 

 If, on the other hand, a cell be suspended in pure water the cell 

 greatly increases in size and finally bursts. This is the case when 

 any solution is separated from pure water or from a less-concen- 

 trated solution by a membrane which to the dissolved substance is 

 impermeable but to the water of .the solution permeable. The solu- 

 tion exerts pressure on the membrane and the water passes through 

 the membrane into the solution. The pressure is called osmotic 

 pressure, and depends not upon the percentage of solute, but upon 

 the number of particles molecules and ions per unit volume. The 

 amount of this pressure varies in different cells, but for mammals 

 it is supposed to be about that of a 0.9 per cent, sodium chlorid 

 (NaCl) solution, since in such a solution the tissue neither gains nor 

 loses weight. This is about 7.1 atmospheres. 



However, some bacteria and many molds can survive and even 

 grow in salt solution which would be fatal to the life of the cell of 

 higher plants. Penicillium and Aspergillus have been known to 

 thrive in solutions, the osmotic pressure of which is equivalent 

 to a 20 per cent, potassium nitrate solution. Bacillus onthracis 

 flourishes on agar containing as much as from 8 to 10 per cent, of 

 sodium chlorid. Since turgidity is essential to growth, it follows 

 that these organisms must have some means of altering the pressure 

 of their cell contents according to the concentration of the sur- 

 rounding medium; only in this way can plasmolysis be avoided. 

 The plasmotic membrane in the case of many bacteria is highly 

 permeable; this would be the case, especially with those organisms 

 which grow in brines. Even some pathogenic bacteria possess the 

 power of accommodating themselves to high osmotic pressures. 

 Bacillus choleras are temporarily plasmolyzed by salt and sucrose 

 solutions but not at all by a glycerin solution, the cell membrane 

 being permeable to the latter. The plasmolysis produced by the 

 salt and sugar disappear in the course of an hour or two as a rule, 

 showing that even salt and sugar slowly penetrate the plasmotic 

 membrane. 



Kind of Food Required. The quality of the food required by 

 bacteria varies greatly with the species. This is well exemplified 

 in Jensen's classification of bacteria which is based upon the sources 

 of nutrition and distinguishes the following groups: 



"1. Bacteria which, like green plants, need neither organic 

 carbon nor organic nitrogen. These so-called ' autotrophic bacteria' 

 can build up both carbohydrates and proteins out of carbon dioxid 

 and inorganic salts. 



"2, Bacteria which need organic carbon compounds, but can 



