Bacillus subtilis cell wall twisting upon cleavage: evidence for stress and strain

Abstract

Bacterial cells exhibit chiral twisting during elongation, a phenomenon previously observed in Bacillus subtilis macrofibers and more recently in Escherichia coli. While twisting has been linked to cell wall mechanics, the precise biophysical mechanisms underlying this behavior remain unclear. Here, we investigate how the balance between cell wall synthesis and hydrolase activity influences twisting in B. subtilis. We propose that torsional stress accumulates within the peptidoglycan network due to circumferentially oriented glycan insertion, which distributes stress on the wall from turgor and is released upon hydrolase-mediated cleavage. Supporting this model, we find that cells deficient in hydrolases twist significantly less and have morphological effects of buckling and coiling, suggesting that cleavage events relieve torsional strain. Increasing Rod complex activity by deleting ponA accelerates twisting, reinforcing the idea that stress accumulates from increased circumferential insertion as the wall expands. Notably, chiral rotation occurs concurrently with cell separation, further demonstrating the presence of stored torsional stress. Unlike E. coli, where MreB filament orientation correlates with twisting, B. subtilis twisting appears independent of MreB motion, suggesting an alternative mechanism rooted in peptidoglycan crosslinking and stress redistribution. Additionally, it is proposed that torsional stress originates from turgor pressure. Our findings establish a direct link between bacterial mechanobiology and cell wall remodeling, highlighting the role of hydrolases in modulating torsional forces during growth.