C9-mediated killing of bacterial cells by transferred C5b-8 complexes: transferred C5b-9 complexes are nonbactericidal.

The formation of the C5b-9 complex on the outer membrane of complement-sensitive cells of Escherichia coli results in inhibition of inner membrane function and the death of the cell. Cells bearing a precursor of the C5b-9 site, the C5b-8 complex, suffer no loss in viability. Antibiotic-sensitive, complement-sensitive donor cells bearing precursor C5b-8 complexes were incubated with equal numbers of antibiotic-resistant, complement-sensitive acceptor cells that had not been exposed to a complement source. This cell mixture was incubated with 5 mM EDTA for 5 min and then with calcium chloride (20 mM) for various times. The excess calcium ion concentration was effectively reduced with additional EDTA, and the cell mixture was washed and resuspended in buffer. The viability of the acceptor cells was assayed by plating on antibiotic-containing media. C9 was added to the mixture, and the mixture was incubated for 10 min at 37 degrees C and then plated as described above. It was found that the acceptor cells were killed by the addition of purified C9 only after incubation with donor cells bearing C5b-8 sites during the transfer procedure. This indicates that precursor C5b-8 sites that support C9-mediated killing could be transferred between cells. No loss in viability was detected for acceptor cells subjected to the procedure described above in the presence of donor cells bearing complete C5b-9 complexes, formed prior to mixing with acceptor cells for the transfer procedure.