Role of Lipopolysaccharide and Capsule in the Serum Resistance of Bacteremic Strains of Escherichia coli

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Abstract
To define the relative roles of capsule and lipopolysaccharide in the virulence of Escherichia coli obtained from blood, we compared the behavior of K1- and K5-encapsulated strains in serum bactericidal and rat virulence assays. Unencapsulated isogenic mutants selected from five parent strains of E. coli O12:K1, but not of O18:K1 or O7:K1 (all rough-specific phage insensitive), were lysed by normal human sera. In contrast, isogenic mutants from strains of serotypes O6:K5 and O18:K5 retained the serum resistance of the parent strains. There was a >105 difference in LD50 in newborn rats between K1-positive and K1-negative pairs of E. coli serotypes O18 and O7 and a >1 log difference between isogenic pairs of serotype 012; however, the K5 isogenic pairs had a similar LD50. Some non-O6 O serotypes, however, required the K5 capsule for serum resistance. We conclude that some O serotypes require encapsulation for optimal virulence but that other O serotypes may not.