Abstract
The importance of K antigens of Escherichia coli as virulence factors was studied by comparing groups of mice given either strains of E. coli isolated from urinary tract infection in humans or mutant strains differing only in the absence of the K antigen. K antigens proved to be of minor importance for mouse nephropathogenicity; however, with the exception of the K(A) antigen, they contributed substantially to deaths attributed to more general infection. Possible mechanisms for the virulence of strains with K antigens are discussed in terms of the bactericidal effect of serum and phagocytosis.