Elasticity and wrinkled morphology of Bacillus subtilis pellicles

Abstract

Wrinkled morphology is a distinctive phenotype observed in mature biofilms produced by a great number of bacteria. Here we study the formation of macroscopic structures (wrinkles and folds) observed during the maturation of Bacillus subtilis pellicles in relation to their mechanical response. We show how the mechanical buckling instability can explain their formation. By performing simple tests, we highlight the role of confining geometry and growth in determining the symmetry of wrinkles. We also experimentally demonstrate that the pellicles are soft elastic materials for small deformations induced by a tensile device. The wrinkled structures are then described by using the equations of elastic plates, which include the growth process as a simple parameter representing biomass production. This growth controls buckling instability, which triggers the formation of wrinkles. We also describe how the structure of ripples is modified when capillary effects are dominant. Finally, the experiments performed on a mutant strain indicate that the presence of an extracellular matrix is required to maintain a connective and elastic pellicle.biofilm elasticity | biofilm growth | wrinkles formation B acterial biofilms most often refer to communities that self-assemble into a cohesive extracellular matrix on solid surfaces or as pellicles floating on top of liquids. Bacteria self-organize in a collective behavior, giving a large-scale coherence to the system. Biofilms thus represent a protected life mode allowing bacteria to survive in hostile environments and from where they can disperse to colonize new niches (1, 2). A primary characteristic of biofilm formation is the production of exopolymeric substances by some cells. These substances mostly consists of exopolysaccharides (EPS), a few specific proteins, and nucleic acids (3-5), but their exact composition depends on the strain of the bacterium and the type of nutrients present in the culture medium (6, 7). When studied in a laboratory, wild-type strains of Bacillus subtilis are known to produce floating pellicles of rich and complex multiscale architectures (8, 9). The vertical structures range from the local 50-μm-scale "fruiting bodies" (10) to the extended macroscopic patterns illustrated in Fig. 1. As recently suggested in ref. 11, multiscale roughness could play a role in increasing the defense capability of B. subtilis against vapor and liquid antimicrobial agents.This study centers on the physical forces acting on biofilm and determining their morphologies at the macroscopic scale. To proceed, we restrict our experimental approach to the simplified case where macroscopic pellicles stand on rich static media. We primarily focus on the pellicles formed by the wild-type strain NCIB 3610, but present some features measured on another wildtype strain DV1 to obtain a more complete picture of existing morphologies. Of course the phenotypes are even more complex in nature given the possible multistrain and species coassembly and the diversity of settings and environments.T...

The Effect of Cell Death on the Stability of a Growing Biofilm

“…Experimental results have shown that during development, wildtype B. subtilis biofilms initially grow in height until reaching a critical thickness at which point vertical growth slows in lieu of horizontal expansion [29,37]. It is also commonly observed that patterning occurs in mature (deep) biofilms [1,5,33]. Thus our analysis is in accordance with observed biological results.…”

“…Biofilm models incorporating additional biological (in particular the role of EPS [18]) and mechanical processes, following similar approaches as explored in e.g. [15,33], may provide a complementary understanding of biofilm pattern formation.…”

“…where B 1 < 0, and is given in (33). It is noted that (36) is of the form h 1 (t) = ω(k, t)h 1 (t), where ω(k, t) is the dispersive coefficient of system (12)- (13) about the planar solutions (19)- (21).…”

“…The growth surface represents agar in a typical laboratory experiment where penetration into the agar is minimal and nutrients within the medium are not limiting for growth (initially in any case). Under these conditions complex biofilm morphologies can be produced, particularly in mature biofilms which are often not of uniform depth and indeed can form elaborate, heterogeneous large scale structures [1,3,5,33] (see Figure 1). We extend the model formulated in [13] by introducing a new cell death term to represent the findings of Asally et al [1], which stated that cell death at the base of biofilms acts as a precursor to wrinkling, specifically within the central region of the biofilm (i.e.…”

Measuring forces and stressesin situin living tissues

“…The use of a standard force sensor in such experiments can reveal the value of the tension [as has been done, for example, with bacterial biofilms, where such force sensors determined that biofilms are under compression rather than tension (Trejo et al, 2013)]. Cutting tissues with a scalpel is obviously only feasible at the macro scale; at smaller scales, lasers can be used.…”