Summary: Na NMR spectroscopy was used to determine free Na concentrations in a halotolerant bacterium, sp., and . The internal Na concentration of both strains depended little on the growth phases and was unchanged after 5 d storage at 2°C. In sp. the level of intracellular sodium increased gradually at higher extracellular NaCI concentrations in both the presence and absence of yeast extract in the growth medium. cells accumulated a higher concentration of free Na than those of sp. The change of Na concentration in both strains was inverse to that of growth rate. When appropriate amounts of osmoprotectants (proline, glycine betaine, or γ-aminobutyrate) were added with the NaCI, internal free Na levels in sp. were lowered, but those of were unchanged. While addition of KCl to medium containing NaCI increased the intracellular level of free Na, the total sodium concentration in the cells remained unchanged, indicating that sodium that had been bound or attached was made free in the cytosol. In sp. grown in the presence of 0.5 M NaCI, free and bound sodium concentrations in the cytosol were estimated to be 0.14 and 0.23 μmol (mg protein), respectively. As a result, visibility by Na NMR was 38%.


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