Energetic basis of development of salt-tolerant transport in a moderately halophilic bacterium, Vibrio costicola

Abstract

Active transport of α-aminoisobutyric acid (AIB) in Vibrio costicola utilizes a system with affinity for glycine, alanine and, to some extent, methionine. AIB transport was more tolerant of high salt concentrations (3–4 M NaCl) in cells grown in the presence of 1.0 M NaCl than in those grown in the presence of 0.5 M NaCl. The former cells could also maintain much higher ATP contents than the latter in high salt concentrations.

Transport kinetic studies performed with bacteria grown in 1.0 M NaCl revealed three effects of the Na⁺ ion: the first effect is to increase the apparent affinity (K _t) of the transport system for AIB at Na⁺ concentrations <0.2 M, the second to increase the maximum velocity (V _max) of transport (Na⁺ concentrations between 0.2 and 1.0 M), and the third to decrease the V _max without affectig K _t (Na⁺ concentrations >1.0 M). Cells grown in the presence of 0.5 M or 1.0 M NaCl had similar affinity for AIV. Thus, the differences in salt response of transport in these cells do not seem due to differences in AIB binding. Large, transport-inhibitory concentrations of NaCl resulted in efflux of AIB from cells preloaded in 0.5 M or 1.0 M NaCl, with most dramatic efflux occurring from the cells whose AIB transport was more salt-sensitive. Our results suggest that the degree to which high salt concentrations affect the transmembrane electrochemical energy source used for transport and ATP synthesis is an important determinant of salt tolerance.