The majority of the matrix protein TapA is dispensable for biofilm formation by Bacillus subtilis

Biofilm formation is a co-operative behaviour where microbial cells become embedded in an extracellular matrix. This biomolecular matrix plays a key role in the manifestation of the beneficial or detrimental outcome mediated by the collective of cells. Bacillus subtilis is an important bacterium for understanding the general principles of biofilm formation and is a plant growth-promoting organism. The protein components of the B. subtilis matrix include the secreted proteins BslA, which forms a hydrophobic coat over the biofilm, and TasA, which forms protease-resistant fibres needed for structuring. A third protein TapA (for TasA anchoring and assembly protein) is needed for biofilm formation and helps TasA fibre formation in vivo but is dispensable for TasA-fibre assembly in vitro. Here we show that TapA is subjected to proteolytic cleavage in the biofilm and that only the first 57 amino acids of the 253-amino acid protein are required for biofilm architecture. However, through the construction of a strain which lacks all eight extracellular proteases, we show that proteolytic cleavage by these enzymes is not a prerequisite for TapA function. It remains unknown why TapA is synthesized at a full length of 253 amino acids when the first 57 are sufficient for biofilm structuring, but the findings do not exclude the core conserved region of TapA having a second role beyond that of structuring the B. subtilis biofilm.