Summary: organisms, grown in basal medium of water activity (a) 0·993 and in basal medium adjusted to several different a values by addition of NaCl, were analysed for the following components: sodium, potassium, calcium, magnesium, total amino acids, inorganic phosphate, chloride, water, DNA, RNA, protein. Cell water decreased from 1·66 to 0·83 g./g. dry weight when a was decreased from 0·993 to 0·90. Internal concentrations of solutes generally increased with decrease in a, potassium and phosphate concentrations being greatest at 0·92 a and sodium, chloride, magnesium, and amino acids at 0·90 a, the lowest a value studied. Increases in potassium and amino acid concentrations resulted largely from the decreased water contents of cocci grown at low a values. Intracellular sodium and chloride concentrations were much lower than and proportional to the NaCl concentration in the medium. The predominant cell solute was potassium at 0·92 a and above, and sodium below 0·92 a. The data are discussed in relation to the inhibition of bacterial growth at low a values.


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