The Assimilation of Amino-acids by Bacteria: 1. The Passage of certain Amino-acids across the Cell wall and their concentration in the internal environment of Streptococcus faecalis

Abstract

SUMMARY: By a method for determining the free amino-acid content of bacterial cells, Streptococcus faecalis (Lancefield Group D) was found to have free lysine, glutamic acid, ornithine and histidine in its internal environment, the amount depending to a certain extent upon the growth medium. The internal lysine and glutamic acid are unable to diffuse out of the cells when these are suspended in water or suitable salt solutions. When the cells are grown in a medium deficient in amino-acids, the internal environment is also deficient and those cells can be used to study the conditions governing the passage of certain amino-acids into the cell. Lysine enters the cell by diffusion, probably passing the cell wall in the isoelectric state. Glutamic acid, glutamine and histidine cannot pass across the cell wall unless energy is supplied by some exergonic metabolism such as the simultaneous fermentation of glucose. Under equilibrium conditions the internal concentration of free lysine or glutamic acid within the cell is markedly greater than that outside; the lower the external concentration, the higher the concentration difference across the cell wall. Glutamic acid inside the cell can pass out into the external environment if fermentation is taking place. The final equilibrium conditions are the same whether the glutamic acid is entering or leaving the cell. Lysine cannot diffuse freely out of the cell, but outward migration takes place if fermentation occurs simultaneously. The uptake of lysine by deficient cells is decreased by the presence of glucose and this effect is, in turn, partially abolished by the uptake of glutamic acid in the presence of glucose.