Circumferential gap propagation in an anisotropic elastic bacterial sacculus

Abstract

We have modelled stress concentration around small gaps in anisotropic elastic sheets, corresponding to the peptidoglycan sacculus of bacterial cells, under loading corresponding to the effects of turgor pressure in rod-shaped bacteria. We find that under normal conditions the stress concentration is insufficient to mechanically rupture bacteria, even for gaps up to a micron in length. We then explored the effects of stress-dependent smart-autolysins, as hypothesised by Arthur L Koch [Advances in Microbial Physiology 24, 301 (1983); Research in Microbiology 141, 529 (1990)]. We show that the measured anisotropic elasticity of the PG sacculus can lead to stable circumferential propagation of small gaps in the sacculus. This is consistent with the recent observation of circumferential propagation of PG-associated MreB patches in rod-shaped bacteria. We also find a bistable regime of both circumferential and axial gap propagation, which agrees with behavior reported in cytoskeletal mutants of B. subtilis. We conclude that the elastic anisotropies of a bacterial sacculus, as characterised experimentally, may be relevant for maintaining rod-shaped bacterial growth.

Importance of being cross-linked for the bacterial cell wall

“…Since MreB directs peptidoglycan insertion [47], it naturally will play an important role in the crack repair process. In fact, it has been proposed that the observed MreB patch propagation in the circumferential direction is due to stable circumferential propagation of small gaps in the anisotropic sacculus [63]. Our model paves the way for more detailed work, leading upto a precise study of the mechanism by which cracks on the cell surface are healed even as the cell is growing.…”