Amyloid fibers provide structural integrity to <i>Bacillus subtilis</i> biofilms

TLDR

Bacillus subtilis biofilms rely on an extracellular matrix, with TasA protein and exopolysaccharide identified as key components. The study aimed to characterize TasA’s physicochemical properties and propose that its amyloid fibers bind cells together in the biofilm. The authors propose that TasA assembles into amyloid fibers that tether cells within the biofilm. Purified TasA forms 10–15 nm wide fibers of variable length that are amyloid, resist harsh treatments, and restore wild‑type biofilm morphology when added to a tasA mutant, confirming its biological activity.

Abstract

Bacillus subtilis forms biofilms whose constituent cells are held together by an extracellular matrix. Previous studies have shown that the protein TasA and an exopolysaccharide are the main components of the matrix. Given the importance of TasA in biofilm formation, we characterized the physicochemical properties of this protein. We report that purified TasA forms fibers of variable length and 10–15 nm in width. Biochemical analyses, in combination with the use of specific dyes and microscopic analyses, indicate that TasA forms amyloid fibers. Consistent with this hypothesis, TasA fibers required harsh treatments (e.g., formic acid) to be depolymerized. When added to a culture of a tasA mutant, purified TasA restored wild-type biofilm morphology, indicating that the purified protein retained biological activity. We propose that TasA forms amyloid fibers that bind cells together in the biofilm.