Complement is a fundamental component of our innate immune system. Its activation, induced by pathogens like bacteria, leads to an irreversible enzymatic cascade, ending with the formation of membrane attack complex (MAC) pores on microbial membranes. This process leads to cell death.
Using atomic force microscopy on bacterial model membranes, we have resolved the formation of individual MAC in real time, revealing a kinetic bottleneck at the insertion of the first C9 monomer to the C5b8 precursor complex in the membrane. To understand how MAC formation occurs in vivo and leads to bacterial cell death, we have also developed protocols to perform AFM experiments in which we resolve MAC pores at the surface of live E. coli.
By a combination of AFM and live-dead fluorescent staining, we can assess MAC formation on these bacteria and relate subsequent cell death to changes at the bacterial surface as induced by MAC pores. Of note, the exact mechanism of anti-bacterial killing remains elusive: in particular, new experiments will be needed to clarify how the lysis occurs, the precise role played by MAC pore location, and the correlation between bacterial outer-membrane integrity and cell death.