Development of Salt-Resistant Active Transport in a Moderately Halophilic Bacterium

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The moderately halophilic bacterium Vibrio costicola accumulates α-aminoisobutyric acid (AIB) by active transport. Substantial amounts of Na+ ions are needed for this transport. This is not due to an ionic requirement for respiration; cells respire as well as KCl as in NaCl but do not transport AIB in KCl. In cells grown in the presence of 1.0 or 2.0 M NaCl, AIB transport took place in higher NaCl concentrations than in cells grown in the presence of 0.5 M NaCl. The latter cells developed salt-resistant transport when they were exposed to 1.0 M NaCl in the presence of chloramphenicol and other antibiotics that inhibit protein synthesis. Two levels of salt-resistant transport were observed. One level (resistance to 3.0 M NaCl) developed in 1.0 M NaCl without the addition of nutrients, did not seem to require an increase in internal solute concentration, and was not lost when cells grown in 1.0 M NaCl were suspended in 0.5 M NaCl. The second level (resistance to 4.0 M NaCl) developed in 1.0 M NaCl only when nutrients were added, may have required an increased internal solute concentration, and was lost when 1.0 M NaCl-grown cells were suspended in 0.5 M NaCl or KCl. Among the substances that stimulated the development of salt-resistant AIB transport, betaine was especially active. Furthermore, direct addition of betaine permitted cells to transport AIB at higher NaCl concentrations. High salt concentrations inhibited endogenous respiration to a lesser extent than AIB transport, especially in 0.5 M NaCl-grown cells. Thus, these concentrations of salt did not inhibit AIB transport by inhibiting respiration. However, oxidation of glucose and oxidation of succinate were at least as sensitive to high salt concentrations as AIB transport, suggesting that a salt-sensitive transport step(s) is involved in the oxidation of these substrates.

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