Role of lipopolysaccharide and capsule in the serum resistance of bacteremic strains of Escherichia coli

A. S. Cross, Kwang Sik Kim, D. C. Wright, J. C. Sadoff, P. Gemski

Research output: Contribution to journalArticle


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 O18 and O7 and a >1 log difference between isogenic pairs of serotype O12; 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.

Original languageEnglish (US)
Pages (from-to)497-503
Number of pages7
JournalJournal of Infectious Diseases
Issue number3
StatePublished - 1986
Externally publishedYes


ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health
  • Immunology

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