On the architecture of the gram-negative bacterial murein sacculus

Abstract

The peptidoglycan network of the murein sacculus must be porous so that nutrients, waste products, and secreted proteins can pass through. Using Escherichia coli and Pseudomonas aeruginosa as a baseline for gram-negative sacculi, the hole size distribution in the peptidoglycan network has been modeled by computer simulation to deduce the network's properties. By requiring that the distribution of glycan chain lengths predicted by the model be in accord with the distribution observed, we conclude that the holes are slits running essentially perpendicular to the local axis of the glycan chains (i. e., the slits run along the long axis of the cell). This result is in accord with previous permeability measurements of Beveridge and Jack and Demchik and Koch. We outline possible advantages that might accrue to the bacterium via this architecture and suggest ways in which such defect structures might be detected. Certainly, large molecules do penetrate the peptidoglycan layer of gram-negative bacteria, and the small slits that we suggest might be made larger by the bacterium.

FIG. 1

(A) Diagram of a portion of a murein sacculus. The hexagons represent the N-acetylglucosamine and the N-acetylmuramic acid (Mur) groups. Two complete cross-links are shown attached to the latter. One is a nonapeptide link (left), while the other is an octapeptide link (right). Circles containing • or × indicate unlinked pentapeptide groups, attached to Mur groups and oriented out of or into the plane. (B and D) Diagram of infinitely long glycan strands (horizontal) maximally cross-linked via nonapeptide or octapeptide chains. A tessera is shown shaded. The dots represent the sugar groups along the glycan strands. (D) Effect of temperature-driven fluctuations in this elastic system. (C and E) The peptidoglycan network of A with six glycan strand ends shown (arrows) giving rise to an aggregate of four tesserae (shaded). The hexagonal structure is not represented here. (E) Effect of temperature-driven fluctuations in this elastic system. The arrow points to a small, attached but isolated peptidoglycan remnant that is relatively free to move. Note the possibility of opening a large hole in the sacculus.

FIG. 2

(A) Diagram of infinitely long glycan strands (horizontal) maximally cross-linked via nonapeptide or octapeptide chains. A tessera is shown shaded. (B) The peptidoglycan system of panel A. The locations where the glycan strands may be cut are indicated by ×s. (C) Construction of an aggregate according to rule B1. The glycan chain has been cut in two places (dark arrows), and the three tesserae thus connected are shaded. (D) Illustration of rule B2. With the glycan chain cut at the dark arrow, the two ×s on either side of the cut (light arrows) have been removed to show that those sites may not be cut. The solid circles represent sugar groups along the glycan strands.

FIG. 3

D_L(L), the fraction of glycan strands possessing length L, as a function of glycan strand length L, for different values of repulsive potential V₀. The dashed lines indicate a V₀ of 0 (short dashes) and a V₀ of 2. The solid lines indicate a V₀ of 5. At an L of 61, the curve indicates the sum of all values of D_L(L) for an L of >60. The insets show typical distributions of aggregates for a V₀ of 5. (A) Use of rule B1. D_s(s) is gaussian, with s₀ equal to 4 and Γ equal to 2. (B to D) Use of rule B2 to eliminate short glycan strands. D_s(s) = 1 for s = s₀ and D_s(s) = 0 for s ≠ s₀; otherwise all aggregates possess the same number of tesserae. (B) s₀ = 2; (C) s₀ = 3; (D) s₀ = 5.

FIG. 4

(A) Turgor pressure-stretched peptidoglycan network such that the holes formed by the tesserae become hexagons. The cross-links at two peptide stem junctions are being cut at the regions marked by ×s to form a slit aggregate comprising three tesserae. (B) The network of A with the slit shown. It can be seen that although the slit is ∼6 nm long, the lateral dimension will remain ∼2 nm.