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

Active transport of -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.Abbreviations AIB -aminoisobutyric acid - pmf proton motive force