Abstract
The human gut microbiome is a dynamic ecosystem where bacteria engage in interspecies competition using molecular weapons such as the type VI secretion system (T6SS). Here, we characterize BteO-BtiO, a unique effector-immunity pair in Bacteroides fragilis that mediates antagonism via a nonenzymatic mechanism. Microscopy reveals that BteO exposure leads to cell elongation, membrane blebbing, and lysis in sensitive strains. Structural and biochemical analyses demonstrate that BteO disrupts cell wall homeostasis by binding to conserved C-terminal domains of OmpA-family proteins (OmpAs), which are critical for outer membrane integrity. The immunity protein BtiO neutralizes BteO by mimicking the OmpA-binding interface. We further show that bile salts enhance BteO-mediated killing in vitro and that BteO confers a competitive advantage in the mammalian gut. Remarkably, BteO exhibits broad-spectrum activity across Bacteroides species. These findings reveal a nonenzymatic strategy of bacterial antagonism and broaden our understanding of T6SS effector diversity within Bacteroides.