K-1 antigen of Escherichia coli: epidemiology and serum sensitivity of pathogenic strains.

K-1 Escherichia coli are far more frequent in neonatal sepsis (36% of E. coli sepsis) and meningitis (80% of E. coli meningitis) than would be expected by the frequency of K-1 E. coli colonization in neonates (11 to 25%). There is no apparent parallel in cases of sepsis in adults. To study further this apparent age-related difference in virulence, E. coli K-1 clinical isolates were tested for their sensitivity to sera. Strains isolated from cases of neonatal meningitis were more sensitive to serum bactericidal activity than those from cases of neonatal or adult sepsis or adult meningitis (P < 0.01). Serum sensitivity did not appear to be determined by K or O antigens. Four isolates sensitive to serum bactericidal activity obtained from neonatal cerebrospinal fluid were killed by adult serum chelated with 0.05 M Mg(2+) ethyleneglycol-bis (beta-aminoethyl ether)-N,N-tetraacetic acid (EGTA), suggesting that the alternative pathway was activated. Although untreated neonatal sera killed these strains as well as adult sera did, EGTA-treated neonatal sera were less effective than EGTA-treated adult sera. This suggests that the alternative pathway function was not activated in neonatal sera. The bactericidal defect of neonatal EGTA-treated serum was partially corrected by addition of either A or B hyperimmune equine meningococcal antiserum.