Surface properties of Streptococcus salivarius HB and nonfibrillar mutants: measurement of zeta potential and elemental composition with X-ray photoelectron spectroscopy

Abstract

To characterize the functional cell surface, the zeta potentials and elemental surface composition of Streptococcus salivarius HB and a range of mutants with known molecular surface structures were determined. Zeta potentials of fully hydrated cells were measured as a function of pH in dilute potassium phosphate solutions, yielding isoelectric points of the strains. Elemental composition (0, C, N, and P) of the outer 2 to 5 nm of the freeze-dried cell surfaces were measured by X-ray photoelectron spectroscopy. An increasing loss of proteinaceous fibrillar surface antigens of the mutants was found to be accompanied by a progressive decrease in the N/C ratio from 0.104 in the parent strain HB to 0.053 in mutant HBC12. Simultaneously, the value of the isoelectric point shifted from 3.0 to 1.3. In a previous study (A. H. Weerkamp, H. C. van der Mei, and J. W. Slot, Infect. Immun. 55:438-455, 1987) on the cell surfaces of the same strains, it was shown that removal of fibrils led to increased exposure of (lipo)teichoic acid at the surface, which explains the low isoelectric point caused by the low pKa of the phosphate groups.The outer surface of bacterial cells mediates many crucial interactions of the cells with their environment, such as adherence, flocculation, antigenicity, susceptibility to phagocytosis, and binding of macromolecules. Understanding these complex functions requires detailed knowledge of the elemental, molecular, and structural composition of the cell surface, which is obviously different from the bulk of the cell wall (3, 18). Generally such knowledge is obtained by electron microscopic methods, by probing the cell surface with specific molecules such as antibodies or lectins, by functional tests such as hydrophobicity assays, or by indirect techniques involving the removal of surface components (8,12).Only a few techniques provide information on the chemical composition on the outer surface of intact cells. An elemental analysis over the thickness of the outer 2 to 5 nm can be obtained by X-ray irradiation of surfaces, causing ejection of electrons, which can be discriminated by their binding energy in a specific chemical element. X-ray photoelectron spectroscopy (XPS) is most frequently used for nonbiological surfaces but has been used in a few cases for the study of bacterial and eucaryotic cells (2, 7, 13). Compared with other surface techniques (1), XPS has the advantage of being nondestructive and highly surface sensitive; it gives a high information content and yields quantitative data on elemental composition and chemical bonding. A disadvantage, common to most analytical surface techniques, is that samples must be measured under high vacuum (10-6 to 10-9 Pa).Previously, we studied the structure and molecular composition of the cell surface of Streptococcus salivarius HB and a range of mutants devoid of specific fibrillar appendages and lacking adhesive abilities (11,17). No information is * Corresponding author. available on the elemental surface composition of these cells and how it w...

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