Non-uniform growth and surface friction determine bacterial biofilm morphology on soft substrates

TL;DR

This study investigates how non-uniform growth and surface friction influence the complex wrinkle patterns in Vibrio cholerae biofilms on soft substrates, combining experiments and a mechanical model.

Contribution

It introduces a comprehensive model linking nutrient diffusion, bacterial growth, mechanical deformation, and friction to explain biofilm morphology patterns.

Findings

Nutrient depletion causes radially-dependent growth and stress patterns.

Increased surface friction shifts wrinkle formation from edges to center.

Model predictions align with experimental wrinkle propagation patterns.

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.