Abstract
Multicellular fibres formed by Bacillus subtilis (B. subtilis) are attracting interest because of their potential application as degradable biomaterials. However, mechanical properties of individual fibres remain unknown because of their small dimensions. Herein, a new approach is developed to investigate the tensile properties of individual fibres with an average diameter of 0.7 μm and a length range of 25.7–254.3 μm. Variations in the tensile strengths of fibres are found to be the result of variable interactions among pairs of microbial cells known as septa. Using Weibull weakest-link model to study this mechanical variability, we predict the length effect of the sample. Moreover, the mechanical properties of fibres are found to depend highly on relative humidity (RH), with a brittle–ductile transition occurring around RH = 45%. The elastic modulus is 5.8 GPa in the brittle state, while decreases to 62.2 MPa in the ductile state. The properties of fibres are investigated by using a spring model (RH < 45%) for its elastic behaviour, and the Kelvin–Voigt model (RH > 45%) for the time-dependent response. Loading-unloading experiments and numerical calculations demonstrate that necking instability comes from structural changes (septa) and viscoelasticity dominates the deformation of fibres at high RH.
Highlights
A controllable hydrodynamic force to bend growing rod-shaped cells of Escherichia coli (E. coli) and B. subtilis to obtain the mechanical stresses regulating the growth of their cell walls
Because the length of a fibre is controlled by various genes related to cell division, we could generate multicellular fibres by deleting sigD, a sigma factor that controls the gene expression of peptidoglycan hydrolase including lytF, lytC, and lytD, and some major peptidoglycan hydrolases such as lytE and lytD7,24,25
The fibres generated by deleting sigD, lytE, and lytD were tested at room temperature and relative humidity (RH) = 28 ± 8%, where 17 fibres were stretched to fracture
Summary
A controllable hydrodynamic force to bend growing rod-shaped cells of Escherichia coli (E. coli) and B. subtilis to obtain the mechanical stresses regulating the growth of their cell walls. We develop a method for studying the mechanical properties of multicellular fibres generated from cell-separation-suppressed mutants of B. subtilis with various genes deleted To our knowledge, this is the first time that the mechanical properties of cellular fibres with slenderness ratios of 37–363 (i.e., several to several hundred bacterial cells) have been directly measured. This is the first time that the mechanical properties of cellular fibres with slenderness ratios of 37–363 (i.e., several to several hundred bacterial cells) have been directly measured These measurements are conducted using a novel manipulation procedure called “liquid drop method (LDM)” on the proposed multilevel measurement platform. This study should provide a possibility to understand the deformation mechanisms of bacterial fibres for further improvements on their performance
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