Molecular organization of Gram-negative peptidoglycan

Abstract

The stress-bearing component of the bacterial cell wall--a multi-gigadalton bag-like molecule called the sacculus--is synthesized from peptidoglycan. Whereas the chemical composition and the 3-dimensional structure of the peptidoglycan subunit (in at least one conformation) are known, the architecture of the assembled sacculus is not. Four decades' worth of biochemical and electron microscopy experiments have resulted in two leading 3-D peptidoglycan models: "Layered" and "Scaffold", in which the glycan strands are parallel and perpendicular to the cell surface, respectively. Here we resolved the basic architecture of purified, frozen-hydrated sacculi through electron cryotomography. In the Gram-negative sacculus, a single layer of glycans lie parallel to the cell surface, roughly perpendicular to the long axis of the cell, encircling the cell in a disorganized hoop-like fashion.

Fig. 1.

Models of peptidoglycan organization. The Layered (A), Scaffold (B), and Disorganized Layered (C) models are viewed obliquely to the sacculus surface and toward the cell pole. Disaccharides (green pills) are approximately 1-nm long and the stretched peptide crosslinks (blue sticks) are approximately 4 nm long. The length distribution of the glycan strands (3-mers to 14-mers) approximates the resolvable part of HPLC chromatograms of purified, hydrolyzed sacculi (10), and the crosslinkage is kept less than the experimentally determined maximum of 50% (21). It is not known whether glycans run parallel or antiparallel to each other.

Fig. 2.

Comparison of in situ and in vitro sacculi. (A) Twenty-five-nm thick Z-slice through the 3-D reconstruction of a C. crescentus CB15N cell. (B) 10-nm thick Z-slice through the 3-D reconstruction of a purified sacculus. Both panels are at the same scale. Abbreviations: (SL) S-layer; (OM) outer membrane; (PG) peptidoglycan layer, that is, the sacculus; (IM) inner membrane; (r) ribosome-like particle; (St) stalk; (Au) gold fiducial; (gr) granule; (gl) glycan strand; (SW) side wall. (Inset) Close-up of sacculus showing glycan strands. Note that the highly flexible sacculus has flattened because of the surface tension of the thin aqueous film, so that it appears wider than the intact cell. The cell tomogram is from reference (16). (C) Enlarged view (4-nm thick slice) showing the glycan strands in the sacculus side wall boxed in white in (B). (D) The same glycan strands as in (C), rotated approximately 90° around the vertical axis.

Fig. 3.

Tomographic slices of E. coli sacculi. Ten-nm thick Z-slices of E. coli strain MG1655 (A) and XL-10 sacculi (B). Abbreviations are the same as in Fig. 2 except for: (w) wrinkle. The double-headed arrow denotes the saccular polar axis. (Insets) Four-fold enlarged views of the boxed regions. Note that the direction of the strand densities is approximately perpendicular to the saccular polar axis. Compared to C. crescentus, E. coli sacculi images had poorer contrast, which may be a result of the ice surrounding E. coli sacculi being at least 20 nm thicker.

Fig. 4.

Details of peptidoglycan organization. (A) 30-nm thick Z-slice of two overlapping C. crescentus CB15N sacculi, outlined by green or violet dotted lines. Abbreviations are the same as in Figs. 3 and 3 except for: (cf) carbon support film. (Insets) 30-nm thick cross-sections cut through the yellow line in the main panel. The yellow dotted line follows the “upper” and “lower” halves of the peptidoglycan. (B) Enlarged view (4-nm thick slice) of the sacculus side wall boxed in (A). (C) Image (B) after 3-D median filtration; one putative glycan strand is highlighted in blue. The green-shaded region demarcates the peptidoglycan. (D) Isosurface rendering the volume shown in (C). The putative glycan strand in (D) is boxed in blue. The noise densities outside of the sacculus are colored gray for clarity. (Inset) Superposition of glycan 9-mer atomic model in the blue-boxed density, shown for a sense of scale.