Abiotic cues modulate exopolysaccharides to influence type VI secretion system-mediated antagonism
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The type VI secretion system (T6SS) enables Gram-negative bacteria to inject toxic effectors into neighboring cells, mediating contact-dependent antagonism and interbacterial competition. How the T6SS-mediated attack responds to environmental cues varies and remains unclear among different bacteria. Here, using Agrobacterium fabrum C58, a soil-borne phytopathogenic bacterium, we investigated the impact of osmolarity, moisture, surface stiffness, and glucose on T6SS-mediated antagonism. We show that these abiotic factors influenced the production of two polysaccharides, cyclic-β-(1,2)-glucan (CβG) and succinoglycan (SG), and modulated the T6SS-killing outcome. Mechanistically, high osmolarity inhibits CβG production, thereby enhancing the expression and secretion of the T6SS. In contrast, SG biosynthesis, in response to moisture, surface stiffness, and glucose, does not impact T6SS expression and function but decreases T6SS-mediated killing efficacy. Electron microscopy revealed that SG creates a physical barrier between bacterial cells. Such physical distancing not only hinders the T6SS attack from Agrobacterium , but also confers protection against other competitors at both intra- and inter-species levels. Our results unravel the complexity of how specific environmental factors modulate the contact-dependent antagonism and highlight a balance between offensive and defensive behaviors.
Significance Statement
Bacteria live in polymicrobial communities where they often need to fight off competitors to survive. One well-characterized weapon is the type VI secretion system (T6SS), a nanomachine mediating contact-dependent antagonism. In this study, we aim to study how the T6SS attack is influenced by environmental cues. We discovered that carbon sources, osmolarity, and surface stiffness modulate the T6SS efficacy through the secretion of sugar chains outside the cell. This sugar secretion increases the physical distance between cells, protecting against T6SS-mediated attack. However, such physical distancing also hinders the efficacy of the T6SS attack originating from Agrobacterium itself. Our results reveal a previously understudied offense-defense tradeoff between EPS production and T6SS-mediated attack. This finding underscores the intricate balance between bacterial offense and defense strategies.
