Abstract
Marine bacteria are unique in the requirement for Na+ for optimal growth. Using a marine bacterium Vibrio alginolyticus, it was confirmed that Na+ is essential for the active uptake of all amino acids. Furthermore, the respiratory chain of V. alginolyticus was found to require Na+ for the maximum activity. The site of Na+-dependent activation is localized in the NADH-quinone reductase segment, where Na+ is extruded from the cells as a direct result of redox reaction. Thus, marine bacteria are able to directly generate sodium-motive force by respiratory chain activity. The sodium-motive force is directly coupled to the active uptake of nutrients and to the rotation of polar flagella. In addition to the energy coupling by proton circulation, marine bacteria are unique in utilizing Na+ circulation for the energy coupling. The latter mode of energy coupling is superior to proton circulation especially at alkaline and Na+-rich conditions. The respiration-coupled Na+ pump is widely distributed among Gram-negative marine and moderately halophilic bacteria. Recently, it was found that the same type of Na+ pump is distributed in the Gram-negative pathogenic bacteria. Since the presence of Na+ pump widens the adaptability of bacteria to grow at harsh environments, Na+ pump is likely to be helpful for the growth of pathogenic bacteria in the host cells to manifest their pathogenicity.
