ABSORPTION OF AMINO-ACIDS 8l 



positive bacteria contained free amino-acids. Moreover, 

 since many amino-acid exacting organisms are also Gram- 

 positive, their presence might be the outcome of mechanisms 

 enabUng such organisms to absorb essential amino-acids 



from their environment. In view of the potential signifi- 

 cance of these observations, Gale decided to undertake a 

 series of more detailed investigations. Apart from the intrinsic 

 value of the results these experiments provide a valuable 

 example of one type of approach to microbiological problems. 



Procedure for assaying internal amino-acids 



A thick suspension (about 30 mg.dry \vt./ml.) of washed 

 cells of the organism is prepared and the amount of external 

 amino-acid is determined by adding the appropriate decar- 

 boxylase preparation to a sample of this suspension. The 

 total amount of free amino-acid, i.e. the amount inside the 

 cells plus that outside, is determined by adding the decar- 

 boxylase to a sample of the suspension which has been 

 previously heated at 100° C. for 15 minutes in order to dis- 

 rupt the cells. The amount of amino-acid inside the cells 

 is then readily calculated by subtracting the former result 

 from the latter. Concentrations are expressed in terms of 

 either the amount of amino-acid in a specified dry weight 

 of cells, or the amount per millilitre of cell volume or 

 'internal free-space'. The volume occupied by the cells was 



