AMINO-ACID SYNTHESIS 99 



These syntlieses have been deduced from studies with 

 artificially-induced biochemical mutants but mutation also 

 occurs spontaneously and, consequently, mutants of bacteria, 

 etc., will arise which have lost the ability to synthesise amino- 

 acids. This is the case with freshly isolated strains of Eberthella 

 typhosa which are unable to synthesise tryptophan and con- 

 sequently cannot grow in a tryptophan-free medium. If a 

 trace of tryptophan is added to the basal medium of salts, 

 ammonia and glucose, then normal growth and subculture is 

 possible. The organisms are able to grow if provided with 

 indole so the enzyme which has been lost catalyses a step in 

 the biosynthesis of indole rather than of tryptophan itself. 

 Fildes has shown that it is possible to select tryptophan- 

 synthesising mutants from the non-synthesising culture and 

 so, presumably, obtain the primitive synthetically competent 

 strain (see p. 63). Eberthella typhosa is exacting, if at all, 

 to tryptophan alone, but other species, especially Gram- 

 positive cocci, are unable to synthesise other amino-acids and 

 consequently will grow only in a medium rich in preformed 

 amino-acids. 



If a cell has lost the ability to synthesise its amino-acids, it 

 necessarily becomes dependent upon the supply of these 

 substances in the medium. The concentration and relative 

 proportions of the various amino-acids in the medium will 

 rarely be those optimal for protein sjmthesis by the organism, 

 and it has been shown recently that certain Gram-positive 

 bacteria, such as the Streptococci and Staphylococci, have 

 acquired a concentration mechanism which compensates for 

 this loss of synthetic ability. These organisms possess a cell- 

 wall or membrane which enables them to take up amino-acids 

 from the external environment and to concentrate them inside 

 the cell prior to metabolism or condensation into protein. 

 Basic amino-acids such as lysine are able to diffuse through 

 this cell-wall, but acidic amino-acids such as glutamic acid 

 cannot penetrate the wall unless energy is supplied to the cell 

 through a metabolic process such as fermentation. If the cell 

 ferments glucose, then glutamic acid passes rapidly through the 



