The effect of sodium chloride on the respiratory activity of a moderately halophilic halotolerant bacterium was studied. Irrespective of growth conditions, resting cells oxidized succinate at a low rate unless sodium chloride was included in the assay mixture, maximum respiratory rates being obtained for sodium chloride concentrations between 0·2m and 0·8m. Neither potassium chloride nor sucrose could replace the sodium salt. The response of the respiratory system to sodium chloride concentration above the optimum depended on growth conditions. Respiration of cells harvested from a low-salt medium was almost inhibited completely by 2·0m-sodium chloride, and that of cells grown and washed in the presence of 2·0m-sodium chloride by 30%. After preincubation with a growth medium containing 2·0m-sodium chloride, even with all multiplication suppressed by chloramphenicol, the resistance of the respiratory system of low-salt-grown organisms to high salt concentrations increased considerably and resembled that of their high-salt-grown counterparts. A similar increase in resistance occurred after preincubation with yeast extract or with choline. With labelled choline, energy-dependent accumulation of labelled material occurred, the conditions required for maximum accumulation and retention being the same as those that increased the salt resistance of the respiratory system. The chromatographic behaviour of the labelled material indicated that the substance was not choline but a derivative, possibly betaine.

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