Biology Reference
In-Depth Information
FIGURE 17.1
Schematic of the
E. coli
cell envelope. Phospholipids are purple and proteins are
yellow. The diglucosamine backbone and inner core of LPS are colored blue while the O antigen is
depicted as green and red. Frequently the first sugar in the O antigen is either GlcNAc or GalNAc
but the polymer can be homopolymeric or heteropolymeric. The capsule, which is green and yellow,
can also be a homo- or heteropolymer. The group 2 and 3 CPSs are known to be anchored in the
membrane by a phospholipid, but it is not known whether the group 1 and 4 CPSs are covalently
anchored to the membrane or just associated with it.
E. coli
isolates also produce an exopolysaccharide called colanic acid which has
a biosynthetic pathway that is part of the Rcs-regulon (
Majdalani and Gottes-
man, 2005
). Unlike CPS, colanic acid is poorly retained at the cell surface. Like
several other bacterial species, the biofilm mode of growth in
E. coli
is supported
by formation of bacterial cellulose and a by-polymer of
N
-acetylglucosamine
(PNAG) (
Cerca and Jefferson, 2008
;
Saldana et al., 2009
). However, some com-
mensal
E. coli
isolates also produce a polysaccharide of unknown structure that
impairs biofilm formation by other bacteria including
Staphylococcus aureus
,
potentially affecting community dynamics (
Rendueles et al., 2011
). Similar anti-
adhesion properties have been reported for certain
E. coli
CPSs (
Valle et al.,
2006
). In summary, the cell surfaces of
E. coli
isolates are rich in complex carbo-
hydrates and these molecules play diverse, niche-dependent roles in the physiol-
ogy of
E. coli
. In this chapter, we will focus only on LPS and CPS.
STRUCTURE AND BIOSYNTHESIS OF
E. COLI
LPS
The LPS molecule contains three regions, the lipid A, core, and O-PS. Lipid A is
the most highly conserved portion of the molecule. The typical structure contains
two phosphorylated glucosamine residues with six acyl chains (
Figure 17.2
A),
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