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. 2003 Dec;85(6):4076-92.
doi: 10.1016/S0006-3495(03)74820-6.

The cell wall of lactic acid bacteria: surface constituents and macromolecular conformations

Prisca Schär-Zammaretti  1 , Job Ubbink
Affiliations

The cell wall of lactic acid bacteria: surface constituents and macromolecular conformations

Prisca Schär-Zammaretti et al. Biophys J. 2003 Dec.

Abstract

A variety of strains of the genus Lactobacillus was investigated with respect to the structure, softness, and interactions of their outer surface layers in order to construct structure-property relations of the Gram-positive bacterial cell wall. The role of the conformational properties of the constituents of the outer cell-wall layers and their spatial distribution on the cell wall is emphasized. Atomic force microscopy was used to resolve the surface structure, interactions, and softness of the bacterial cell wall at nanometer-length scales and upwards. The pH-dependence of the electrophoretic mobility and a novel interfacial adhesion assay were used to analyze the average physicochemical properties of the bacterial strains. The bacterial surface is smooth when a compact layer of globular proteins constitutes the outer surface, e.g., the S-layer of L. crispatus DSM20584. In contrast, for two other S-layer containing strains (L. helveticus ATCC12046 and L. helveticus ATCC15009), the S-layer is covered by polymeric surface constituents which adopt a much more extended conformation and which confer a certain roughness to the surface. Consequently, the S-layer is important for the overall surface properties of L. crispatus, but not for the surface properties of L. helveticus. Both surface proteins (L. crispatus DSM20584) and (lipo)teichoic acids (L. johnsonii ATCC332) confer hydrophobic properties to the bacterial surface whereas polysaccharides (L. johnsonii DSM20533 and L. johnsonii ATCC 33200) render the bacterial surface hydrophilic. Using the interfacial adhesion assay, it was demonstrated that hydrophobic groups within the cell wall adsorb limited quantities of hydrophobic compounds. The present work demonstrates that the impressive variation in surface properties displayed by even a limited number of genetically-related bacterial strains can be understood in terms of established colloidal concepts, provided that sufficiently detailed structural, chemical, and conformational information on the surface constituents is available.

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Figures

FIGURE 1
FIGURE 1
The ζ-potential of the L. johnsonii strains as a function of pH in a 10-mM potassium phosphate buffer. All data points are the average of two measurements with independently fermented cultures. L. johnsonii DSM20533 (triangles); L. johnsonii ATCC332 (diamonds); and L. johnsonii ATCC33200 (circles). Error bars are not shown, as they are generally smaller than the symbols.
FIGURE 2
FIGURE 2
The ζ-potential of the S-layer containing strains harvested in the late-stationary and exponential growth phases. The bacteria are suspended in a 10-mM KH2PO4 buffer. Late-stationary phase (unfilled symbols); logarithmic phase (solid symbols). L. crispatus DSM20584 (triangles); L. helveticus ATCC12046 (circles); and L. helveticus ATCC15009 (diamonds). Error bars are shown, but are generally smaller than the symbols for bacteria harvested in late-stationary phase.
FIGURE 3
FIGURE 3
Interfacial adhesion curves of the six strains in 10 mM potassium phosphate buffer at pH = 7. Plotted are the ratio of the volume of hexadecane to aqueous buffer on the horizontal axis, and the fraction of microorganisms adhered at the water-hexadecane interface on the vertical axis. L. johnsonii DSM20533 (solid triangles); L. johnsonii ATCC332 (solid diamonds); L. johnsonii ATCC33200 (solid circles); L. crispatus DSM20584 (unfilled triangles); L. helveticus ATCC12046 (unfilled circles); and L. helveticus ATCC15009 (unfilled diamonds). Initial cell count of the buffers is such that the OD is between 0.45 and 0.55. All data points are the average of three measurements with three independently fermented cultures. Error bars denote ±1 SD.
FIGURE 4
FIGURE 4
AFM deflection images showing the surface morphology of the Lactobacillus strains. The microorganisms are adhered to a poly-L-lysine-covered substrate. Imaging is performed in a 10 mM KH2PO4 buffer at pH 7. (a) L. johnsonii DSM20533; (b) L. johnsonii ATCC332; (c) L. johnsonii ATCC33200; (d) L. crispatus DSM20584; (e) L. helveticus ATCC12046; and (f) L. helveticus ATCC15009. The surfaces of the L. johnsonii strains exhibit a fuzzy character, whereas the definition of the surfaces of the S-layer-containing strains is higher, and the surfaces appear smoother.
FIGURE 5
FIGURE 5
Force-distance curves showing the interactions of the AFM tip with the bacterial surfaces. (a) L. johnsonii DSM20533; (b) L. johnsonii ATCC332; (c) L. johnsonii ATCC33200; (d) L. crispatus DSM20584; (e) L. helveticus ATCC12046; and (f) L. helveticus ATCC15009. The microorganisms are adhered to a poly-L-lysine covered substrate. The force-distance curves are obtained in a 10 mM KH2PO4 buffer at pH 7. The L. johnsonii strains (a–c) show clear adhesion peaks upon retraction of the AFM tip from the bacterial surface. The adhesion curves of L. johnsonii DSM20533 and L. johnsonii ATCC332 show multiple unbinding events upon retraction, whereas for L. johnsonii ATCC33200 the unbinding appears to proceed via a single event. For the L. crispatus DSM20584, no significant adhesion between the bacterial surface and the AFM tip is observed upon retraction; adhesion events are also rarely recorded for the two L. helveticus strains. From the force-distance curves, elasticity data are calculated (Table 4).
FIGURE 6
FIGURE 6
Transmission electron micrographs of the Lactobacillus strains harvested in late-stationary phase. (a) L. johnsonii DSM20533; (b) L. johnsonii ATCC33200; and (c) L. crispatus DSM20584. In all three images, a dark band can be observed ∼20–40 nm below the surface (white arrows). The dark staining hints at a high protein content. Only in the case of L. crispatus DSM20584 (c), a thin dark band can be seen at the outer surface (black arrow and insert). It is inferred that this protein-rich layer largely determines the surface properties of the strain. Bar = 250 nm. Magnification of the insert is 2.2× the magnification of the main image.
FIGURE 7
FIGURE 7
Elasticity maps of Lactobacillus strains harvested in late stationary phase. (a) L. johnsonii DSM20533; (b) L. johnsonii ATCC33200; and (c) L. crispatus DSM 20584. The microorganisms are adhered to a poly-L-lysine covered substrate. The elasticity data are obtained in a 10 mM KH2PO4 buffer at pH 7. The elasticity is plotted on a relative scale from 0 to 1 (AU, arbitrary units). Comparison of the surface stiffness of the various bacteria is possible as the stiffness of the poly-L-lysine adsorbed on the substrate slides serves as a reference. The elasticity of the surface of L. crispatus DSM 20584 is high and spatially fairly homogeneous (c). The surfaces of the two L. johnsonii strains are much softer and the surface elasticity is heterogeneous (a and b).
FIGURE 8
FIGURE 8
AFM-characterization of the poly-L-lysine substrates. (a) Force-distance curves. (b) Distribution of adhesion forces as determined from the minima of the force-distance curves. (c) Relative elasticity according to the FIEL method. In the analysis for b and c, 1024 force-distance curves were used.
FIGURE 9
FIGURE 9
Structural models of the bacterial cell wall of the Lactobacillus strains, utilized in the interpretation of the various experiments. (1) Cell membrane. (2) Inner, protein-rich layer of the cell wall. (3) Outer layer of the cell wall, rich in various polymers like polysaccharides and lipoteichoic acids. (4) Extracellular polysaccharides and other polymeric compounds attached to the cell wall protruding into the buffer. (5) Surface proteins (S-layer); even if the surface proteins form a close packing, a significant fraction of the surface is open to the outside. (6) Crosslinked polymer layer outside of the layer containing the surface proteins.
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