Structural determination of the O-chain polysaccharide from the haloalkaliphilic Halomonas alkaliantarctica bacterium strain CRSS.

In hypersaline environments there are plenty of microorganisms belonging to both Bacteria and Archaea domains. These extremophiles have developed biochemical adaptations which comprise the accumulation of molar concentrations of potassium and chloride and the biosynthesis and/or the accumulation of organic osmotic solutes (osmolytes) within the cytoplasm. Moreover, to maintain the turgor of the cells halophiles enhance the production of anionic phospholipids and alter the fatty acid composition of the membrane lipids, but very little is known about adaptational structural changes of the lipopolysaccharides (LPS), the main constituent of the outer leaflet of the outer membrane of Gram-negative bacteria. The aim of this work is to investigate the chemical structure of these LPS in order to provide insight into the adaptation mechanism of halophiles to live at high salt concentration. For this, Halomonas alkaliantarctica, a haloalkaliphilic Gram-negative bacterium isolated from salt sediments of a saline lake in Cape Russell in the Antarctic continent, was cultivated and the LPS were extracted and analysed. The structure of the O-chain of the LPS from H. alkaliantarctica was determined by chemical analysis, 1-D and 2-D NMR spectroscopy. The polysaccharide was constituted of a linear trisaccharidic repeating unit as follows: -->3)-beta-l-Rhap-(1-->4)-alpha-l-Rhap-(1-->3)-alpha-l-Rhap-(1--> A comparison among the O-chain structures of H. alkaliantarctica and other Halomonas species is also reported.