Capsule and lipopolysaccharide - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 519

Chapter 17
Capsule and lipopolysaccharide
Lisa M. Willis and Chris Whitfield
University of Guelph, Guelph, ON, Canada
INTRODUCTION
Pathogenic E. coli require many different virulence factors which allow them
to invade the host, evade host immune defenses and colonize specific niches in
the host where they can cause disease. The first interactions between E. coli and
its host occur at the outer membrane and are mediated by proteins and carbo-
hydrate-containing macromolecules (glycoconjugates) on the bacterial and host
cell surfaces ( Figure 17.1 ). Bacterial glycoconjugates provide crucial defenses
against different elements of the innate and acquired immune system, as well as
generating tremendous diversity in surface antigenicity. In most Gram-negative
bacteria, the outer membrane is composed predominantly of the glycolipid
known as lipopolysaccharide (LPS). In E. coli , this complex molecule is com-
posed of three structurally distinct regions; the hydrophobic anchor called lipid
A, a core oligosaccharide, and the long-chain polysaccharide called O antigen
(or O-polysaccharide; O-PS). The differences in E. coli O-PS structures give
rise to more than 180 distinct O antigens and these have been exploited in sero-
typing classification methods ( Orskov et al., 1977 ). Many E. coli isolates also
produce another long-chain polysaccharide, known as capsular polysaccharide
(CPS). CPS provides another major surface antigen, called the K-antigen, named
after the German term 'kapsel'. There are more than 80 different K antigens in
E. coli . The means of attachment of CPS to the cell surface is not known in all
cases but these polymers create a coherent structural entity (the capsule) that is
visible by light microscopy and extends 50-100 nm from the cell surface. As
a result, the capsule often masks underlying O antigens in serotyping studies
that exploit specific antisera and whole-cell agglutination methods. A single
E. coli isolate can produce one O- and one K-antigen.
Together, LPS and CPS represent major virulence factors, which have been
the target of numerous vaccines and therapies, and they provide the focus of
this chapter. However, they are not the only glycoconjugates produced by
E. coli . All isolates produce a polysaccharide called enterobacterial common
antigen that provides resistance to organic acids in E. coli ( Barua et al., 2002 ) and
to bile salts and detergents in Salmonella ( Ramos-Morales et al., 2003 ). Under
certain stress conditions (e.g. perturbations of cell envelope integrity), some