The role of betaine as a factor influencing the salt resistance of the respiratory system in resting cells of the moderately halophilic halotolerant bacterium Ba1 was studied. Betaine accelerated succinate oxidation in cells obtained from low-salt medium, and stimulation of the respiratory rate was stronger the higher the sodium chloride concentration in the assay medium. The stimulatory effect also depended on the ratio of betaine concentration to the amount of bacteria present. Accumulation of labelled betaine by the bacterial cells was demonstrated; like the respiratory stimulation, it was favourably influenced by an increase in the sodium chloride concentration of the medium. In cells harvested from a high-salt medium and washed with 2·0m-sodium chloride, betaine caused no increase in the respiratory rate, nor was the already high salt resistance of the respiratory system further improved by the addition of betaine. When, however, these cells lost their salt resistance as a result of washing in the absence of sodium chloride, betaine was able to restore it to its original level. In contrast with respiration in low-salt-grown bacteria, that in high-salt-grown cells was not affected by betaine, even after they were washed in the absence of sodium chloride, when the sodium chloride concentration was optimum.

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