Application of X-ray photoelectron spectroscopy to micro-organisms

The use of X-ray photoelectron spectroscopy (XPS) for the analysis of microbial cell surfaces is described and discussed. This technique is well established in materials science; however, its application to biological samples calls for special considerations owing to the complexity of their chemical composition and because living cells are delicate samples needing careful handling. The results presented for various microorganisms demonstrate how data on the elemental composition can be upgraded into information on the functional groups by peak decomposition and how this can in turn be indicative of the molecular constituents present at the cell surface. The reproducibility of the analysis of microbial surfaces is shown to be currently better than 10% for major elements and functions; it is mainly determined by the variability of the biological material. The reliability of the results is examined with respect to the possibility of systematic errors and the influence of experimental conditions. The effects of washing of the cells, freeze drying, storage and manipulation for introduction into the spectrometer are checked. Sample degradation under the X-ray beam is examined. The influence of surface contamination and of uncertainties concerning sensitivity factors is discussed on the basis of biochemical standard compound analysis. Different sets of data show the dependence of surface composition on the nutrient availability (nature of the culture medium, yeast extract concentration, phosphate concentration), culture age and physiological state (vegetative cells, spores) and its relation to surface morphology. Relationships are observed between the surface electrical properties and the phosphate or nitrogen surface concentrations, depending on the type of microorganism. The chemical composition parameter (nitrogen, oxygen, carbon bound only to carbon and hydrogen) related to surface hydrophobicity depends on the type of microorganism. The surface composition is also found to be relevant to the behaviour of microbial cells with respect to interfaces (flotation, aggregation, adhesion to a solid support)