Synergistic effect of cefotaxime and normal human serum on bactericidal activity against urinary strains of Escherichia coli with capsular antigen K1.

About 11% of Escherichia coli strains isolated from patients with urinary tract infections produced capsular antigen K1. In a group of strains resistant to normal human serum the percentage of strains with K1 antigen was higher than that without this antigen. Cefotaxime and serum showed synergistic action in the bactericidal reaction against E. coli strains resistant to the serum. Lack of synergistic effect of thermally inactivated serum with subminimal concentrations of cefotaxime indicated participation of complement in this reaction. The antibiotic used in subminimal concentrations caused significant elongation of E. coli bacilli.