Serum requirements necessary for the opsonophagocytosis ofescherichia coli O73:K92:H1

TheEscherichia coli O73:K92:H1 serotype, which possesses a capsular antigen immunochemically similar to the capsule of the group C meningococcus, is demonstrated in this study to be resistant to phagocytosis by normal human PMNs and serum and to be dependent upon immune antibody and presumably the classical complement pathway for opsonization. Using both a luminol-dependent chemiluminescence assay and an in vitro bactericidal system, we examined, in both the absence and presence of complement, the opsonic activity of IgM ang IgG antisera. Of the various antisera tested, only those sera cross-reactive with the K92 capsular antigen were found to be opsonic both in vivo and in vitro, while somatic O or lipid A antisera demonstrated no activity. In in vitro studies with capsular IgM and IgG antisera, only IgG demonstrated opsonic activity without complement, whereas IgM required complement for opsonization of O73:K92:H1. These data demonstrate that antisera directed toward capsular antigens are opsonic for this phagocytosis-resistantE. coli, and that complement is a necessity for opsonization in the absence of sufficient capsular IgG antibodies.