Non-linear stress-softening of the bacterial cell wall confers cell shape homeostasis.

The bacillus - or rod - is a pervasive cellular morphology among bacteria. Rod-shaped bacteria elongate without widening by reinforcing their cell wall anisotropically, along the cell's circumference, but it is unknown how cells adaptively tune anisotropy to homeostatically control cell width. Through super-resolution measurements of cell wall mechanical properties, we discovered that the cell wall exhibits non-linear stress-softening exclusively in the circumferential direction. Furthermore, during steady-state growth the cell wall is inflated precisely to the acute non-linear transition. Physics-based theory correctly predicted that this transition underlies the negative feedback that governs cell width homeostasis. In other words, the cell wall is a "smart material" whose exotic mechanical properties are exquisitely adapted to execute cellular morphogenesis.