The Composition of Staphylococcus Aureus in Relation to the Water Activity of the Growth Medium.

Staphylococcus aureus 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 h, potassium and phosphate concentrations being greatest a t 0-92 a, and sodium, chloride, magnesium, and amino acids a t 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.