Phagocytic resistance of Escherichia coli K-1 isolates and relationship to virulence.

Blood culture isolates from 133 episodes of Escherichia coli bacteremia were typed for K-1 capsular antigen by immunodiffusion, utilizing equine antiserum raised against meningococcal group B polysaccharide. Twenty-six percent (34 of 133) of these isolates were positive for K-1 antigen. These 133 strains, 34 K-1 and 99 non-K-1, were tested for susceptibility to phagocytosis. K-1 strains were found to be more resistant to clearance (27%) than non-K-1 strains (71%) when tested in an in vitro opsonophagocytic/killing assay containing normal human granulocytes and plasma. Additional studies demonstrated that resistance was due to decreased phagocytosis rather than diminished intraleukocytic killing. K-1 strains obtained from stool showed a similar degree of resistance to phagocytosis when compared with K-1 blood isolates. A comparison of clinical data on non-neonatal patients with E. coli K-1 and non-K-1 bacteremia showed no significant differences in mortality for these two groups. The incidence of shock for patients bacteremic with K-1 strains (74%) was significantly greater than that for patients bacteremic with non-K-1 strains (33%). These differences are attributed to the increased resistance to phagocytosis observed for K-1 versus non-K-1 E. coli isolates.