Bacterial infections in coagulating blood: calcium inhibits complement alternative pathway

Bacterial survival in coagulated blood can cause complications associated with endocarditis or biomaterial infections. We report that during recalcification of diluted (1:5) citrated human whole blood, the killing activity toward E. coli decreased 90-fold after the addition of thrombin (1 U/ml) and CaCl₂ (50 mM). The killing activity in diluted citrated whole blood or plasma was Mg²⁺-dependent and Ca²⁺-independent (as well as heat-inactivated in plasma), demonstrating complement alternative pathway mediation of E. Coli lysis. Addition of calcium from 0 to 10 mM CaCl₂ in the presence of heparin (5 U/ml), PPACK (250 μM), or hirudin (33 U/ml) to diluted citrated whole blood or plasma caused a dose-dependent inhibition of bacterial killing. Excess addition of fibrin or fibrinogen had no effect on bacterial survival in whole blood or plasma, with or without coagulation. Physiological levels of Ca²⁺ (1 mM) had a cytoprotective effect that attenuated alternative pathway killing of E. coli. This previously unrecognized calcium modulation of the alternative pathway killing must be considered in the study of bacterial survival in coagulating blood