Nonuniform growth and surface friction determine bacterial biofilm morphology on soft substrates

Abstract

During development, organisms acquire three-dimensional shapes with important physiological consequences. While the basic mechanisms underlying morphogenesis are known in eukaryotes, it is often difficult to manipulate them in vivo. To circumvent this issue, here we present a study of developing Vibrio cholerae biofilms grown on agar substrates in which the spatiotemporal morphological patterns were altered by varying the agar concentration. Expanding biofilms are initially flat, but later experience a mechanical instability and become wrinkled. Whereas the peripheral region develops ordered radial stripes, the central region acquires a zigzag herringbone-like wrinkle pattern. Depending on the agar concentration, the wrinkles initially appear either in the peripheral region and propagate inward (low agar concentration) or in the central region and propagate outward (high agar concentration). To understand these experimental observations, we developed a model that considers diffusion of nutrients and their uptake by bacteria, bacterial growth/biofilm matrix production, mechanical deformation of both the biofilm and the agar, and the friction between them. Our model demonstrates that depletion of nutrients beneath the central region of the biofilm results in radially-dependent growth profiles, which in turn, produce anisotropic stresses that dictate the morphology of wrinkles. Furthermore, we predict that increasing surface friction (agar concentration) reduces stress anisotropy and shifts the location of the maximum compressive stress, where the wrinkling instability first occurs, toward the center of the biofilm, in agreement with our experimental observations. Our results are broadly applicable to bacterial biofilms with similar morphologies and also provide insight into how other bacterial biofilms form distinct wrinkle patterns.The intricate shapes of organisms are determined by the spatiotemporal patterns of growth as well as the mechanical properties of their underlying biological components [1-3]. Three-dimensional (3D) shape transformations in developing organisms often arise via differential growth of connected tissues [1, 4]. Such asymmetric growth patterns generate compressive stresses within the faster growing tissues, which may cause mechanical instabilities [5][6][7]. Growth-induced mechanical instabilities drive the formation of many convoluted morphologies, such as the gyrification of brains [2, 8, 9], the vilification and looping of guts [10, 11], and the branching of lungs [12] as well as 3D structures of synthetic systems with patterned swelling [5,[13][14][15][16].Biofilms, which are surface-associated bacterial communities encapsulated by a self-produced extracellular matrix [17, 18], also display a variety of 3D developmental morphologies ranging from radial stripes, to concentric rings, to disordered labyrinth and herringbone patterns [19][20][21][22][23]. In the case of Vibrio cholerae, a model * wingreen@princeton.edu † andrej@princeton.edu biofilm former, quantitative imaging revealed a 3...

Hierarchical transitions and fractal wrinkling drive bacterial pellicle morphogenesis

“…Finite Element Simulation of V. cholerae Pellicles. We use the previously described framework of elastic growth (10,11,18) to model the primary wrinkling instability of V. cholerae pellicles. In brief, we use the deformation gradient tensor F = ∂x=∂X to describe the local shape change of a 2D thin film, in which x = [x,z] T and X = [X,Z] T denote the current coordinates and the material coordinates, respectively.…”

“…These multicellular communities are crucial in contexts such as medical infections and industrial biofouling because cells in biofilms and pellicles display enhanced resilience to antibiotics, immune clearance, and physical perturbations compared to their isogenic planktonic counterparts (7)(8)(9). Analogous to eukaryotic systems, bacterial biofilms and pellicles develop striking macroscopic morphologies including wrinkles and delaminations that are driven by combined biological, materialphysics, and mechanical determinants (10)(11)(12)(13)(14). Studies of biofilms at the single-cell level show the emergence of internal cell ordering (15)(16)(17) and collective cellular flow (18).…”

“…The Cauchy stresses σ associated with the elastic deformation F e are then computed from the elastic constitutive relation of the material. Here, we model the pellicle as an almost incompressible neo-Hookean elastic material (10,11,18) whose strain energy density in the material coordinate system is given by (46) 2 , where G denotes the shear modulus, v ≈ 0.5 denotes the Poisson's ratio,…”

How do biofilms feel their environment?

“…When a bacteria makes contact with a surface, it initiates a program of gene expression that promotes colonization and biofilm formation ( 1, 2 ). As the biofilm grows, it absorbs nutrients from its environment and generates internal forces that allow the biofilm to expand and move against its substrate ( 3-6 ). Biofilm growth thus depends on chemical signals but also mechanical interactions with its environment.…”

Cell differentiation and motion determine the Bacillus subtilis biofilm morphological evolution under the competitive growth

“…Despite their ancient evolutionary origins, bacteria can exhibit complex competition patterns and morphological, especially when they live in communities called biofilms [1,2]. Biofilms are surface-associated bacterial communities encapsulated by a self-produced extracellular matrix, and a predominant growth mode of bacteria in nature [3][4][5]. Biofilms are involved in aspects many of our lives, such as medical care [6], effluents purifying [7], biofuel cells [8].…”