We have a new article out in PNAS featuring former student Salem Al Mosleh as first author.

Abstract. Bacterial growth is remarkably robust to environmental fluctuations, yet the mechanisms of growth-rate homeostasis are poorly understood. Here, we combine theory and experiment to infer mechanisms by which Escherichia coli adapts its growth rate in response to changes in osmolarity, a fundamental physicochemical property of the environment. The central tenet of our theoretical model is that cell-envelope expansion is only sensitive to local information, such as enzyme concentrations, cell-envelope curvature, and mechanical strain in the envelope. We constrained this model with quantitative measurements of the dynamics of E. coli elongation rate and cell width after hyperosmotic shock. Our analysis demonstrated that adaptive cell-envelope softening is a key process underlying growth-rate homeostasis. Furthermore, our model correctly predicted that softening does not occur above a critical hyperosmotic shock magnitude and precisely recapitulated the elongation-rate dynamics in response to shocks with magnitude larger than this threshold. Finally, we found that, to coordinately achieve growth-rate and cell-width homeostasis, cells employ direct feedback between cell-envelope curvature and envelope expansion. In sum, our analysis points to cellular mechanisms of bacterial growth-rate homeostasis and provides a practical theoretical framework for understanding this process.

Why is this important? The bacterial cell envelope is the critical structure that defines cell size and shape, and its expansion therefore defines cell growth. Although size, shape, and growth rate are important cellular variables that are robust to environmental fluctuations, the feedback mechanisms by which these variables influence cell-envelope expansion are unknown. Here, we explore how Escherichia coli cells achieve growth-rate and cell-width homeostasis during fluctuations in osmolarity, a key environmental property. A biophysical model in which the cell envelope softens after an osmotic shock and envelope expansion depends directly on local curvature quantitatively recapitulated all experimental observations. Our study elucidates new mechanisms of bacterial cell morphogenesis and highlights the intimate interplay between global cellular variables and the mechanisms of cell-envelope expansion.

New paper called “Thermal Fluctuations of Singular Bar-Joint Mechanisms”

Manu Mannattil, J. M. Schwarz, and Christian D. Santangelo, Phys. Rev. Lett. 128, 208005 – Published 20 May 2022

A bar-joint mechanism is a deformable assembly of freely rotating joints connected by stiff bars. Here we develop a formalism to study the equilibration of common bar-joint mechanisms with a thermal bath. When the constraints in a mechanism cease to be linearly independent, singularities can appear in its shape space, which is the part of its configuration space after discarding rigid motions. We show that the free-energy landscape of a mechanism at low temperatures is dominated by the neighborhoods of points that correspond to these singularities. We consider two example mechanisms with shape-space singularities and find that they are more likely to be found in configurations near the singularities than others. These findings are expected to help improve the design of nanomechanisms for various applications.