Serum antibodies to outer membrane proteins of Escherichia coli in healthy persons and patients with bacteremia

Antibodies to Escherichia coli outer membrane proteins in sera from healthy persons and from patients bacteremic with various enteric or nonenteric bacteria were measured by an enzyme-linked immunosorbent assay (ELISA). Outer membranes were prepared from E. coli O55. Serum was absorbed with E. coli O55 lipopolysaccharide and diluted 1:100 for immunoglobulin A (IgA) or IgM and 1:1,000 for IgG antibodies. Paired serum specimens were obtained from the 56 patients included in the study (the first specimen on the day of positive blood culture and the second specimen 8 to 12 days later) and compared with sera from blood donors (n = 50) as controls. On an average, the patients bacteremic with enterobacteria (n = 40) showed increased levels of antibodies of all three immunoglobulin classes in the first serum specimens and significantly higher levels in the second specimens compared with the controls, although with considerable case-to-case variation. Increased levels of IgG antibodies showed the best combination of diagnostic specificity (100%) and sensitivity (53%) for bacteremia caused by enteric bacilli. Mostly, the antibody response was directed against the major E. coli O55 outer membrane proteins at 81,000, 38,500, 33,500, and 7,500 molecular weights as shown by Western blot (immunoblot) analysis. Some of the patients bacteremic with nonenteric bacteria showed increased levels of IgA antibodies, but not of IgG or IgM antibodies. Cross-reactivity of the nonenteric blood culture isolates with the E. coli outer membrane preparation was not demonstrated. The cross-reactivity of the E. coli O55 outer membrane proteins with those of enteric bacilli of other genera was examined by absorption experiments. Western blots with serum absorbed with live E. coli O55 provided evidence that the epitopes of the outer membrane protein at 7,500 molecular weight were available for antibody binding at the bacterial surface, and that at least some of the epitopes of the 38,500- and 33,500-molecular -weight proteins were accessible to antibodies. The results suggest that an ELISA for the measurement of antibodies against cross-reactive outer membrane proteins from enteric bacilli may be useful in the diagnosis of serious infections caused by members of the family Enterobacteriaceae, and that antibodies to the major outer membrane proteins may have an immunobiological function.