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Role of bacterial phospholipases in serum-mediated killing of Escherichia coli

The importance of bacterial phospholipases during serum-mediated killing of Escherichia coli was examined by using wild-type DR+ DS+ and an isogenic phospholipase-deficient mutant DR- DS-. No difference in serum sensitivity was observed when the parental DR+ DS+ and mutant DR- DS- strains were exposed to various concentrations of normal guinea pig serum. Examination of the free fatty acid (FFA) and lipid composition during serum-mediated killing of the two E. coli strains indicated that FFA release occurred only in the parental DR+ DS+ strain. No FFA release or lipid degradation was detected in the mutant DR- DS- strain during serum killing. The addition of heat-inactivated E. coli antiserum (rabbit) to normal guinea pig serum caused FFA release in both E. coli strains. This FFA release was found to be independent of serum-mediated killing and due to a highly active and heat-resistant rabbit serum phospholipase that hydrolyzed the bacterial lipids after serum killing. The data presented indicate that serum-mediated killing of E. coli is independent of FFA release and that activation of bacterial phospholipases and the resulting release of FFA are only a result rather than a cause of serum-mediated cell death.