Figure 2. Cell shape is maintained by uniform peptidoglycan insertion.

(A) Schematic of insertion steps for new glycan strands: (1) An existing peptide crosslink (boxed in orange) is chosen as the insertion starting point (glycans shown in green, peptides in red); (2) Peptides are removed from the adjacent glycan strands until the free path exceeds a defined length; (3) a new glycan strand (blue) is inserted; (4) glycan disaccharide subunits are crosslinked to nearby uncrosslinked glycans (red dots on the new strand indicate peptides that are too far to form crosslinks); (5) the new network is relaxed to its lowest energy state. (B,E) Schematic of random (B) and uniform (E) insertion models, with blue shading and peptide thickness indicating likelihood of selection as insertion site. (C,F) Cell walls grown in silico to six times their original length. (C) When new insertion sites are chosen at random along the surface, severe bending and bulging results. (F) Uniform azimuthal and longitudinal density of insertion-site selection maintains rod shape. (D,G) Local width relative to the cell midline for cells shown in C,F. Line thickness is proportional to cell length. Inset in D shows the cell midline for the cyan-boxed cell in C. Inset in G shows the average width during elongation of the cell in F.