K-1 Antigen Content and the Presence of an Additional Sialic Acid-Containing Antigen Among Bacteremic K-1 Escherichia coli: Correlation with Susceptibility to Opsonophagocytosis

Eighty percent of blood culture isolates of Escherichia coli K-1 are resistant to in vitro opsonophagocytosis by normal human granulocytes and fresh serum. To determine the basis for susceptibility to phagocytosis in 20% of bacteremic K-1 E. coli, we investigated possible quantitative and qualitative immunochemical differences in the K-1 antigen content among resistant and sensitive isolates. We prepared extracts of blood culture K-1 E. coli by sonication and determined the K-1 polysaccharide content per dry weight of bacteria by rocket immunoelectrophoresis using cross-reactive equine anti-group B meningococcal sera. We assessed qualitative differences in the antigen content by crossed immunoelectrophoresis, using an immune globulin fraction and isolated immunoglobulin G (IgG) and IgM from the group B antisera. Three different resistant K-1 isolates contained a mean K-1 content of 48.5 ± 7.6 μg/mg ± standard deviation of dry bacteria, and three sensitive isolates contained 23.2 ± 5.6 μg/mg (P < 0.005). Crossed immunoelectrophoresis of extracts from both sensitive and resistant strains revealed a secondary sialic acid-containing antigen that was electrophoretically different from both the major K-1 antigen and a reference group B meningococcal antigen. This negatively charged secondary antigen was susceptible to Clostridium perfringens neuraminidase degradation and reacted only with IgG whereas the major K-1 antigen reacted only with IgM. This antigen was detected in the extracts of resistant isolates only at 10¹⁰ but not at 10⁹ colony-forming units per milliliter. This study demonstrates that (i) the degree of phagocytosis of bacteremic E. coli K-1 isolates is inversely associated with K-1 content, and (ii) more easily phagocytosed (sensitive) K-1 isolates have greater amounts of an additional sialic acid-containing antigen that appears to be unrelated to the previously described O acetyl K-1 antigen.