Screening for cross-reacting capsular polysaccharide K antigens of Escherichia coli using antiserum agar.

Agar plates containing antiserum against group B meningococcus or Haemophilus influenzae type b were used to determine the prevalence of cross-reacting K1 and K100 capsular polysaccharide antigens in 265 isolates of disease-causing Escherichia coli. K1 antigen was found in 22% of isolates from various sites. K100 antigen was found in only three isolates. This technique is a convenient method to detect specific E. coli K antigens for evaluation as possible factors important in the virulence of the organism.