Nascent biofilms on soft surfaces

TL;DR

This study investigates how the stiffness of soft surfaces influences bacterial colony formation, revealing that softer substrates lead to slower, anisotropic biofilms, while harder surfaces promote rapid, isotropic monolayer expansion, with implications for biofilm engineering.

Contribution

The paper combines experiments and modeling to demonstrate how substrate elasticity controls bacterial biofilm morphology and dynamics, highlighting the role of anisotropic drag forces on soft surfaces.

Findings

Softer surfaces promote multilayered, anisotropic colonies.

Harder surfaces lead to rapid, isotropic monolayer expansion.

Surface compliance influences early biofilm development.

Abstract

Soft surfaces, spanning vastly different environmental and biomedical settings, are frequently colonised by surface-associated bacteria. Yet, how soft surfaces govern bacterial dynamics and their self-organisation into colonies remains poorly understood. Using experiments and agent-based modelling, we report the self-organisation of bacterial cells into nascent biofilms on soft substrates. By tuning the elastic modulus over two orders of magnitude, we show that the colony morphology, spreading dynamics and collective behaviour depend on the substrate stiffness, wherein softer surfaces promote slowly expanding, geometrically anisotropic, multilayered colonies, while harder substrates drive rapid, isotropic expansion of bacterial monolayers before multilayer structures emerge. Supported a cell mechanical model and two-dimensional agent-based simulations, our results identify that anisotropic drag forces on soft substrates, emerging due to local deformations, underpin colony anisotropy and swift verticalisation. In contrast, reduced drag on hard surfaces allows rapid expansion of monolayers, thus delaying the transition to a multilayer structure. Surface compliance, a key but overlooked determinant of early-stage biofilm development, could be harnessed to engineer biofilm structures and dynamics for nature-inspired and biomedical applications.