Effects of Physical Forces on Bacteria 167 



incoming and the outgoing water, and the greater, therefore, will 

 be the osmotic pressm-e within the bag. If the bag is connected to 

 an upright tube of mercury, called a manometer, the pressure 

 built up within the bag will be reflected by a rise in solution in the 

 manometer. This pressure is expressed in terms of millimeters of 

 mercury. 



Protein molecules and colloidal particles are generally large and 

 occupy considerable space compared to crystalloids. Consequently 

 in equimolar concentrations of crystalloids and colloids, there will 

 be fewer colloids per unit volume. This will be reflected in a 

 lower osmotic pressure for colloids. 



The statement is often made that some bacteria, particularly 

 pathogens, when transferred to distilled water will die in a 

 relatively few hours. The inference is made that death is due 

 to osmotic pressure changes. While it is true that water will enter 

 these suspended cells and will build up some pressure, there is 

 more evidence that death of these cells is caused by the presence 

 of toxic trace elements in the water originating in the still, or by the 

 alkali dissolved from the soft glass of the container in which the 

 bacteria are being held. Freshly distilled water is neutral in re- 

 action, but upon standing it absorbs carbon dioxide from the atmos- 

 phere, and in the absence of a buffer, the water may become quite 

 acid. Some bacteria, on the other hand, may be shown actually to 

 multiply in distilled water. 



Imbibition is the word used to designate the taking in of water 

 by protoplasm or gels. Is this different from osmosis? Gortner 

 ( 1937 ) reported that dry seeds are able to withdraw water from a 

 saturated solution of lithium chloride (osmotic pressure of 1,000 

 atmospheres), although the seeds' salt content is only sufficient to 

 account for but a few atmospheres of osmotic pressure. It is 

 probably by this or some similar mechanism that bacterial spores 

 take up water and germinate, and that certain bacteria are able 

 to survive and multiply in concentrated brine solutions. 



Every organism has its maximum, optimum, and minimum con- 

 centration of salt in which it may live and grow, just as each cell 

 has similar limitations with respect to temperature, pH, etc. 



