BslA is a self-assembling bacterial hydrophobin that coats the Bacillus subtilis biofilm

Abstract

Biofilms represent the predominant mode of microbial growth in the natural environment. Bacillus subtilis is a ubiquitous Gram-positive soil bacterium that functions as an effective plant growth-promoting agent. The biofilm matrix is composed of an exopolysaccharide and an amyloid fiber-forming protein, TasA, and assembles with the aid of a small secreted protein, BslA. Here we show that natively synthesized and secreted BslA forms surface layers around the biofilm. Biophysical analysis demonstrates that BslA can self-assemble at interfaces, forming an elastic film. Molecular function is revealed from analysis of the crystal structure of BslA, which consists of an Ig-type fold with the addition of an unusual, extremely hydrophobic “cap” region. A combination of in vivo biofilm formation and in vitro biophysical analysis demonstrates that the central hydrophobic residues of the cap are essential to allow a hydrophobic, nonwetting biofilm to form as they control the surface activity of the BslA protein. The hydrophobic cap exhibits physiochemical properties remarkably similar to the hydrophobic surface found in fungal hydrophobins; thus, BslA is a structurally defined bacterial hydrophobin. We suggest that biofilms formed by other species of bacteria may have evolved similar mechanisms to provide protection to the resident bacterial community.

The Bacterial Hydrophobin BslA is a Switchable Ellipsoidal Janus Nanocolloid

“…4,12 Our simulations provide reliable estimates of the adsorption free energy of wild-type (wt) BslA and enable us to rationalize a number of experimental observations.…”

“…It has been shown previously that purified BslA 42−181 spontaneously aggregates into a stable protein film at air-water interfaces in vitro. 4 Electron microscopic images of the BslA film have demonstrated that the most stable assembly of BslA surface layers correlates with a high degree of order between individual proteins. 12 In our simulations, the outward facing conformation (wt-BslA-L out ) exhibits the most clearly defined upright orientation at the interface in combination with the highest adsorption free energy.…”

“…The deviation from clearly defined upright orientations we show here for mutant proteins is thus likely to be an important factor in the relative loss of mutant BslA layer stability in vitro, and of the observed wettability of the mutant biofilms in vivo. 3,4 A better structural understanding of the stability of the hydrophobic protective layers around biofilms is essential to aid the design of agents that disrupt this huge permeation barrier to water-soluble biocides such as antibiotics.…”

“…3,4 Its properties are underpinned by the recently determined crystal structure of BslA, in which a hydrophobic cap region, which exposes an unusually extended area of hydrophobic surface to external medium, is appended to a large polar domain 4 ( Fig. 1A,B).…”

Self-regulation of exopolysaccharide production in Bacillus subtilis by a tyrosine kinase

“…The genes for fiber production (a three-gene operon consisting of tapA, sipW, and tasA) and EPS production (the 15-gene epsA-O operon) are held silent in growing, motile cells by the SinR and AbrB repressors but are derepressed during biofilm formation, leading to high-level production of the protein and polysaccharide components of the extracellular matrix (Chu et al 2006;Chai et al 2008). An additional component of the matrix, BslA, confers hydrophobicity on the biofilm (Kobayashi and Iwano 2012;Hobley et al 2013). Because mutants blocked in matrix production can be complemented extracellularly by matrix-producing cells (Branda et al 2001), the matrix can be thought of as a common good that is shared among cells in the community.…”