Dual functionality of the TasA amyloid protein in Bacillus physiology and fitness on the phylloplane

Abstract

Bacteria can form biofilms that consist of multicellular communities embedded in an extracellular matrix (ECM). Previous studies have demonstrated that genetic pathways involved in biofilm formation are activated under a variety of environmental conditions to enhance bacterial fitness; however, the functions of the individual ECM components are still poorly understood. In Bacillus subtilis, the main protein component of the ECM is the functional amyloid TasA. In this study, we demonstrate that beyond their well-known defect in biofilm formation, ΔtasA cells also exhibit a range of cytological symptoms indicative of excessive cellular stress, including DNA damage accumulation, changes in membrane potential, higher susceptibility to oxidative stress, and alterations in membrane dynamics. Collectively, these events can lead to increased programmed cell death in the colony. We show that these major physiological changes in ΔtasA cells are likely independent of the structural role of TasA during amyloid fiber formation in the ECM. The presence of TasA in cellular membranes, which would place it in proximity to functional membrane microdomains, and mislocalization of the flotillin-like protein FloT in ΔtasA cells, led us to propose a role for TasA in the stabilization of membrane dynamics as cells enter stationary phase. We found that these alterations caused by the absence of TasA impair the survival, colonization and competition of Bacillus cells on the phylloplane. Taken together, our results allow the separation of two complementary roles of this functional amyloid protein: i) structural functions during ECM assembly and interactions with plants, and ii) a physiological function in which TasA, via its localization to the cell membrane, stabilizes membrane dynamics and supports more effective cellular adaptation to environmental cues.