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 Agrobacteriumfabrum 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-{beta}-(1,2)-glucan (C{beta}G) and succinoglycan (SG), and modulated the T6SS-killing outcome. Mechanistically, high osmolarity inhibits C{beta}G 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.
Wang, Y.-C., Sung, L.-K., Wu, C.-F., Chang, J. H., Ting, S.-Y., Lai, E.-M.
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