68 GENERAL CONDITIONS FOB GROWTH 



isms by using up the oxygen apparently keep the tension 

 low, and this explains how such organisms grow in the soil.^ 



OSMOTIC PRESSURE. 



Like all living cells bacteria are very susceptible to changes 

 in the density of the surrounding medium. If placed in a 

 medium less concentrated than their own protoplasm water 

 is absorbed and they "swell up;" while if placed in a denser 

 medium, water is given off and they shrink (plasmoptysis 

 or plasm olysis) . Should these differences be marked or 

 the transition be sudden, the cell walls may even burst and 

 the organisms be destroyed. If the differences are not 

 too great or if the transition is made gradually, the organ- 

 isms may not be destroyed, but will either cease to grow and 

 slowly die out, or will show very much retarded growth, or 

 will produce abnormal cell forms. This is illustrated in the 

 laboratory in attempting to grow bacteria on food material 

 which has dried out. A practical application of osmotic 

 effects is in the use of a high percentage of sugar in preserv- 

 ing fruits, etc., and in the salting of meats. Neither the 

 cane-sugar nor the common salt themselves injure the bac- 

 teria chemically, but by the high concentration prevent their 

 development. 



ELECTRICITY. 



Careful experimenters have shown that the electric cm-- 

 rent, either direct or alternating, has no direct destructive 

 effect on bacteria. In a liquid medium the organisms may 

 be attracted to or repelled from one or the other pole or 

 may arrange themselves in definite ways between the poles 

 (galvanotaxis), but are not injured. However, electricity 

 through the secondary effects produced, may be used to 

 destroy bacteria. If the passage of the electric current 

 increases the temperature of the medium sufficiently, the bac- 

 teria will be killed, or if injurious chemical substances are 



' It is popularly supposed that, in canning fruit, vegetables, meats, etc., 

 all the air must be removed, since the organisms which cause "spoiling" 

 cannot grow in a vacuum. The existence of anaerobic and facultative 

 anaSrobic bacteria shows the fallacy of such beliefs. 



