X-ray photoelectron spectroscopy analysis of the surface composition of Azospirillum brasilense in relation to growth conditions

X-ray photoelectron spectroscopy (XPS) has been used to determine the surface chemical composition of the Gramnegative bacterium Azospirillum brasilense as a function of growth conditions. As opposed to growth in complex medium, growth in synthetic medium led to low reproducibility of XPS results, and did not show a significant variation of the cell surface composition as a function of culture time; moreover, the molecular composition obtained by modelling the XPS data was found to be generally poorer in proteins, and richer in polysaccharides and in hydrocarbon-like compounds. The direct correlation between the phosphate concentration and the concentration of carbon bound only to carbon and hydrogen, ---(C,H), indicated that hydrocarbon-like compounds detected at the surface of cells grown in synthetic medium are essentially phospholipids. The poor reproducibility of the XPS data obtained for cells grown in synthetic medium and the random observation of phospholipids at the outermost cell surface are not due to cell disruption or to migration of intracellular components during sample preparation; exposure of phospholipids at the cell surface reflects either actual variations of the composition of the native surface induced by nutrient limitation or reorganization of cell surface polymers upon freeze drying.