Mechanism of lateral cell-wall expansion at a constant diameter in Bacillus subtilis

In Escherichia coli, lateral cell-wall expansion during growth occurs by cross-linking of new glycan strands to the existing peptidoglycan network. However, it is unclear whether the same mechanism applies to other rod-shaped bacteria. Here, we use cell imaging and mass spectrometry analysis of isotopically labeled peptidoglycan to study this process in the Gram-positive bacterium Bacillus subtilis. We show that new glycan strands are cross-linked exclusively to other newly synthesized glycan chains, and not to the existing peptidoglycan network. We propose that new peptidoglycan meshes, assembled at the membrane surface, impose a shift on older meshes toward the bacterial surface, where they sustain the cytoplasmic turgor pressure, before their eventual degradation. This outward movement would result in preferential lateral expansion due to the large difference in the strain tensors of the peptidic and glycosidic peptidoglycan components.