Osmotic spreading of Bacillus subtilis biofilms driven by an extracellular matrix

Abstract

Bacterial biofilms are organized communities of cells living in association with surfaces. The hallmark of biofilm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In Bacillus subtilis, secretion of the exopolysaccharide (EPS) component of the extracellular matrix is genetically coupled to the inhibition of flagella-mediated motility. The onset of this switch results in slow expansion of the biofilm on a substrate. Different strains have radically different capabilities in surface colonization: Flagella-null strains spread at the same rate as wild type, while both are dramatically faster than EPS mutants. Multiple functions have been attributed to the EPS, but none of these provides a physical mechanism for generating spreading. We propose that the secretion of EPS drives surface motility by generating osmotic pressure gradients in the extracellular space. A simple mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of biofilm growth, thickening, and spreading. We discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced fitness of the matrix-deficient mutant strains.collective motility | gel swelling | surface translocation | bacterial biofilm | polymeric secretion B acterial biofilms are heterogeneous populations of differentiated bacteria that live in association with surfaces and exhibit a remarkable degree of spatio-temporal organization (1-3). The formation of a mature biofilm occurs in several stages, starting from the attachment of a single cell to a solid substrate. When cells commit to the surface, a protein-and sugar-rich polymeric extracellular matrix (ECM) is secreted in the extracellular space and holds the community together. Several different functions have been attributed to the ECM, ranging from protection to mechanical integrity and reserve of nutrient (4, 5). At the same time, flagella are downregulated, and most cells lose their individual motility. For the Gram-positive soil bacterium Bacillus subtilis the loss of flagella-mediated motility is genetically coupled to the production of extracellular matrix (6-10). This switch results in a slow kind of surface motility that allows the biofilm to spread outward on the substrate. Although spreading has been attributed to a qualitative concept of "pushing" associated with biomass growth, the physical force generating mechanism that drives biofilm expansion outward across a surface is not known. One intriguing possibility is the potential contribution of the ECM to biofilm growth; the ECM is a highly visco-elastic and sticky substance, and it might be expected to hinder expansion rather than facilitate it (4, 5). However, the ECM clearly plays a crucial role in biofilm, and its possible effect in the actual expansion of the biofilms has never been investigated.Here, we demonstrate that, in the first 24 h of biofilm development, extracellular matrix product...

Mechanically-driven phase separation in a growing bacterial colony

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Surfing of bacterial droplets: Bacillus subtilis sliding revisited

“…EPSs are known to facilitate both colony biofilm expansion (8) and sliding by generating osmotic pressure in the extracellular space (6); however, these previous works are not mentioned.…”

Biomechanical Analysis of Infectious Biofilms

“…An alternative reduced geometry is the lateral expansion of circular biofilms by osmosis (i.e. uptake of fluid into the matrix), for which good quantitative agreement between experiments and continuum modelling is possible [81]. Enhanced models have demonstrated that elastic instabilities can generate wrinkles, even without confinement [10,30] .…”