Antibody-independent interaction of the first component of complement with Gram-negative bacteria

The interaction of the first component of complement with two serum-sensitive strains of Escherichia coli and Klebsiella pneumoniae was studied. It could be demonstrated that highly purified C1, free of immunoglobulin G and immunoglobulin M, binds to E. coli or K. pneumoniae. C1 binding was also found with specifically absorbed human serum, after incubation of bacteria with normal serum in the presence of ethylenediaminetetraacetate or agammaglobulinemic serum; the number of C1 molecules taken up by the bacteria was not influenced, indicating that C1 binding was independent of naturally occurring antibodies. C1 bound to bacteria was still able to cleave C4, the natural substrate of C1. From these observations, it is concluded that C1 in an enzymatically active state can be bound directly to bacteria independently of antibody.