Permeability of Microorganisms to Inorganic 

 Ions, Amino Acids and Peptides 



DEAN B. COWIE AND RICHARD B. ROBERTS 



Department of Terrestrial Magnetism, Carnegie 

 Institution of Washington, Washington, D. C. 



DURING THE PAST FIVE YEARS, the group al our laboratory has been using 

 a wide variety of radioactive tracer molecules to study the pathways 

 and rates of biosynthesis in E. coli, strain B.' In these studies we have re- 

 peatedly observed that in addition to material metabolically incorporated into 

 the constituents of the cell, a pellet of cells invariably contains a predictable 

 minimum quantity of the radioactive tracer. Thus, when a gram wet weight 

 of cells is suspended in a radioactive solution and centrifuged, the pellet con- 

 tains at least the radioactivity of 0.75 ml of the suspending fluid. When such 

 a pellet is dried, it loses approximately 0.75 ml of water. Accordingly, we 

 have come to the conclusion that a wet pellet of E. coli cells weighing i gram 

 contains 0.75 ml of water, and this water in turn has the same concentration 

 of ions and molecules as the suspending fluid. 



The only exceptions observed have occurred when very large radioactive 

 molecules (proteins) were used in the suspending fluid. In this case the con- 

 tent of the pellet was equivalent to o.i to 0.2 ml of the suspending fluid. We 

 have attributed this smaller quantity to protein dissolved in the intercellular 

 fluid of the pellet leaving .55 to .65 ml of the fluid which must be within the 

 cell wall. 



In addition to the radioactive material which appears to be simply dissolved 

 in the water of the cell, further material may be incorporated or absorbed. 

 Fortunately, a clear distinction can be made. The quantity of dissolved ma- 

 terial is proportional to the concentration in the suspending fluid; its entrance 

 does not depend on conditions (temperature, composition of medium, etc.) 

 which affect the metabolic activity, and it can be readily washed out. It appears, 

 therefore, that small molecules and ions penetrate the cell wall readily in 

 either direction, and the 'water space' (12) of the cells contains the same 

 concentration of small molecules and ions as the surrounding medium. We 

 prefer to limit the use of the terms 'permeable' and 'permeability' to the simple 

 problem of penetration of the cell wall. In many cases a suspected intermediate 



^ Obtained from the Department of Genetics, Carnegie Institution of Washington. 



