Complement-mediated killing of Escherichia coli by mechanical destabilization of the cell envelope

Complement proteins eliminate Gram-negative bacteria in the blood via the formation of membrane attack complex (MAC) pores in the outer membrane. However, it remains unclear how outer membrane poration leads to inner membrane permeation and cell lysis. Using atomic force microscopy (AFM) on living Escherichia coli ( E. coli ), we probed MAC-induced changes in the cell envelope and correlated these with subsequent cell death. Initially, bacteria survived despite the formation of hundreds of MACs that were randomly distributed over the cell surface. This was followed by larger-scale disruption of the outer membrane, including propagating defects and fractures, and by an overall swelling and stiffening of the bacterial surface, which precede inner membrane permeation. We conclude that bacterial cell lysis is only an indirect effect of MAC formation; outer membrane poration leads to mechanical destabilization of the cell envelope, reducing its ability to contain the turgor pressure, leading to inner membrane permeation and cell death. Synopsis Complement proteins form membrane attack complex pores in the outer membrane of Gram-negative bacteria, leading to bacterial lysis via an incompletely understood mechanism. In this study, atomic force microscopy identifies a pathway of bacterial killing caused by membrane attack complex pore formation in the Gram-negative bacterial outer membrane. Atomic force microscopy shows formation of membrane attack complex pores on living E. coli cells. Initial pore formation leads to propagating defects and fractures in the outer membrane. Resulting cell death follows after overall mechanical destabilization of the cell envelope. Atomic force microscopy analysis shows that the large-scale disruption of the E. coli outer membrane by membrane attack complex pores causes cell swelling and subsequent inner membrane damage.