Membrane changes induced by exposure of Escherichia coli to human serum

The effect of bactericidal concentrations of lysozyme-free human serum on parameters of membrane integrity has been studied in serum-susceptible and serum-resistant Escherichia coli strains. Serum treatment released all of the alkaline phosphatase from the periplasmic space of two rapidly serum-susceptible strains but did so at different rates. In contrast, no periplasmic enzyme was released from two serum-resistant strains or from one moderately susceptible smooth strain. Lysozyme-free serum and heat-inactivated serum released comparable amounts of 86Rb+ from preloaded cells at comparable rates, regardless of serum susceptibility. Serum decreased the rate of phospholipid biosynthesis in both serum-susceptible and serum-resistant strains. In susceptible but not in resistant strains, intracellular ATP pools were depleted after serum exposure. Outer membranes and cytoplasmic membranes were prepared from serum-treated E. coli, and assays for C3 and C5b-9(m) were performed. With rapidly susceptible strains, C3 deposition on the outer membrane without attachment of C5b-9(m) occurred during the short prekilling phase. Subsequent bacterial killing was accompanied by deposition of C5b-9(m), which was recovered with C3 exclusively in outer membrane fractions with increased density and by eventual total loss of recoverable cytoplasmic membranes. Minimal deposition of complement components, without accompanying cytoplasmic membrane loss, occurred with serum-resistant strains. Loss of recoverable cytoplasmic membrane was not due to the action of either serum or bacterial phospholipase A. The results raise the possibilities that C5b-9(m) primarily damages the outer membrane and that the bacteria themselves actively participate in the ensuing, as yet unclarified, metabolic reactions that finally lead to their death.