Structure, assembly and regulation of expression of capsules in Escherichia coli

Mol Microbiol. 1999 Mar;31(5):1307-19. doi: 10.1046/j.1365-2958.1999.01276.x.


Many Escherichia coli strains are covered in a layer of surface-associated polysaccharide called the capsule. Capsular polysaccharides represent a major surface antigen, the K antigen, and more than 80 distinct K serotypes result from structural diversity in these polymers. However, not all capsules consist of K antigen. Some are due to production of an extensive layer of a polymer structurally identical to a lipopolysaccharide O antigen, but distinguished from lipopolysaccharide by the absence of terminal lipid A-core. Recent research has provided insight into the manner in which capsules are organized on the Gram-negative cell surface, the pathways used for their assembly, and the regulatory processes used to control their expression. A limited repertoire of capsule expression systems are available, despite the fact that the producing bacteria occupy a variety of ecological niches and possess diverse physiologies. All of the known capsule assembly systems seen in Gram-negative bacteria are represented in E. coli, as are the majority of the regulatory strategies. Escherichia coli therefore provides a variety of working models on which studies in other bacteria are (or can be) based. In this review, we present an overview of the current molecular and biochemical models for capsule expression in E. coli. By taking into account the organization of capsule gene clusters, details of the assembly pathway, and regulatory features that dictate capsule expression, we provide a new classification system that separates the known capsules of E. coli into four distinct groups.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • ATP-Binding Cassette Transporters / physiology
  • Bacterial Capsules / chemistry
  • Bacterial Capsules / physiology*
  • Bacterial Proteins / physiology
  • Bacterial Translocation
  • Escherichia coli / physiology*
  • Escherichia coli Proteins*
  • Hexosyltransferases / physiology
  • Klebsiella pneumoniae / genetics
  • Models, Biological
  • Models, Genetic
  • O Antigens / biosynthesis
  • Polysaccharides / biosynthesis
  • Polysaccharides, Bacterial / classification
  • Polysaccharides, Bacterial / genetics
  • Polysaccharides, Bacterial / physiology*
  • Transcription Factors*


  • ATP-Binding Cassette Transporters
  • Bacterial Proteins
  • Escherichia coli Proteins
  • O Antigens
  • Polysaccharides
  • Polysaccharides, Bacterial
  • RcsB protein, E coli
  • Transcription Factors
  • RcsB protein, Bacteria
  • RcsA protein, E coli
  • colanic acid
  • Hexosyltransferases
  • O-antigen polymerase