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

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Abstract
Many (80%) blood culture isolates of E. 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, possible quantitative and qualitative immunochemical differences in the K-1 antigen content among resistant and sensitive isolates were investigated. Extracts of blood culture K-1 E. coli were prepared by sonication and the K-1 polysaccharide content per dry weight of bacteria was determined by rocket immunoelectrophoresis using cross-reactive equine anti-group B meningococcal sera. Qualitative differences in the antigen content were assessed by crossed immunoelectrophoresis, using an immune globulin fraction and isolated 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 .mu.g/mg .+-. SD of dry bacteria and 3 sensitive isolates contained 23.2 .+-. 5.6 .mu.g/mg (P < 0.005). Crossed immunoelectrophoresis of extracts from sensitive and resistant strains revealed a secondary sialic acid-containing antigen that was electrophoretically different from the major K-1 antigen and a reference group B meningococcal antigen. This negatively charged secondary antigen was susceptible to Clostridium perfringens neuramindase degradation and reacted only with IgG, 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/ml. The degree of phagocytosis of bacteremic E. coli K-1 isolates is inversely associated with K-1 content. 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.