Measuring the stiffness of bacterial cells from growth rates in hydrogels of tunable elasticity

Abstract

Summary Although bacterial cells are known to experience large forces from osmotic pressure differences and their local microenvironment, quantitative measurements of the mechanical properties of growing bacterial cells have been limited. We provide an experimental approach and theoretical framework for measuring the mechanical properties of live bacteria. We encapsulated bacteria in agarose with a user-defined stiffness, measured the growth rate of individual cells, and fit data to a thin-shell mechanical model to extract the effective longitudinal Young's modulus of the cell envelope of Escherichia coli (50–150 MPa), Bacillus subtilis (100–200 MPa), and Pseudomonas aeruginosa (100–200 MPa). Our data provide estimates of cell wall stiffness similar to values obtained via the more labor-intensive technique of atomic force microscopy. To address physiological perturbations that produce changes in cellular mechanical properties, we tested the effect of A22-induced MreB depolymerization on the stiffness of E. coli. The effective longitudinal Young's modulus was not significantly affected by A22 treatment at short time scales, supporting a model in which the interactions between MreB and the cell wall persist on the same time scale as growth. Our technique therefore enables the rapid determination of how changes in genotype and biochemistry affect the mechanical properties of the bacterial envelope.

A dynamically assembled cell wall synthesis machinery buffers cell growth

“…In addition, transient association loosens the requirement for an MreB complex to spatially and temporally order the steps of cell wall synthesis. This line of reasoning is supported by the observation that growth rate is unaffected by A22 treatment (25), despite disruption of MreB spatial organization. In vitro interactions between E. coli PBP2 and PBP1a (19) and between Helicobacter pylori PBP2 and MreC (20) have been identified, and in the latter case these proteins appear to form a complex in vivo.…”

Getting into shape: How do rod-like bacteria control their geometry?

“…In Gram-negative E. coli, at least one important synthesising enzyme, the transpeptidase PBP2, moves rapidly and diffusively, showing no processivity on the sub-second time scale (Lee et al 2014), thus suggesting a more transient interaction of the cell-wall synthesis proteins. Tuson et al (2012) finds that the timescales at which disrupting MreB affects cell wall elasticity are similar to the growth time, in consistence with this interpretation. Furthermore, filaments have recently been reported to move with a filament-length dependent speed Olshausen et al (2013).…”

Substrate stiffness regulates B‐cell activation, proliferation, class switch, and T‐cell‐independent antibody responses in vivo

“…LPS is present in the outer membrane of Gram-negative bacteria cells. Interestingly, it is known that the Young's modulus values from cells from different types of bacteria are quite diverse, ranging from 50 kP to 769 MPa [39]. Thus, B cells are very likely to experience "stiffer" versus "softer" bacteria cells in vivo.…”