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In-Depth Information
EHEC, although the genes encoding HCP are ubiquitous among
E. coli
strains
(
Xicohtencatl-Cortes et al., 2007
).
BFP fibers are produced from an operon of 14 genes encoded on the large
(∼90 kb) adherence factor plasmid (pEAF) (
Nataro et al., 1987
;
Stone et al.,
1996
). The BFP is a confirmed virulence factor (
Bieber et al., 1998
) that medi-
ates the initial stages of adherence to the host intestinal epithelium (
Cleary
et al., 2004
;
Hyland et al., 2006a
;
Zahavi et al., 2011
). CFA/III fibers are
peritrichous pili that are 5-10 µm long and are often encoded on a large plas-
mid (
Honda et al., 1984
;
Shinagawa et al., 1993
). Some ETEC strains produce
the Longus pilus, so named because it can grow over 20 µm in length, which
is essentially allelic to CFA/III (
Girón et al., 1994
). The Longus T4P display
a polar distribution on the cell surface (
Girón et al., 1994
). The N-termini of
the major pilin proteins in each system, CofA and LngA, and of all acces-
sory genes are nearly identical (
Taniguchi et al., 1995
;
Gomez-Duarte et al.,
2007
). Despite their similarity, these pilus systems are expressed under differ-
ent conditions and by different serogroups of ETEC (
McConnell et al., 1989
;
Girón et al., 1995
). The R64 pilus is encoded on a large conjugative plasmid.
Unlike its counterparts in EPEC and ETEC, the R64 pilus has not been shown
to have a role in adherence but rather is specialized for bacterial conjugation
in liquid media (
Yoshida et al., 1998
). The HCP can grow to over 10 µm in
length and is involved in adherence of EHEC to host cells (
Xicohtencatl-
Cortes et al., 2007
).
GENETIC ORGANIZATION
The genetic organization of T2S systems is relatively well conserved. For all
T2S systems there are 12 conserved genes, termed A-O, although in some cases
additional genes may be required for T2S function (
Peabody et al., 2003
;
Fil-
loux, 2004
). These genes are most often found in a single operon with slight
variations depending on species (
Sandkvist, 2001
). The majority of T2S sys-
tems are located on the chromosome with the exception of a plasmid-encoded
T2S system in
E. coli
O157 (
Cianciotto, 2005
).
Most T4P expressed in
E. coli
are T4bP and thus are found in a continuous
operon on a large plasmid (
Figure 13.1
). The exception, the HCP pilus char-
acterized in EHEC, is a T4aP and is thus encoded on several small operons in
the
E. coli
genome, though little is known about the function of each gene in
these operons outside of
hcpA
(also known as
ppdD
), the main pilus subunit
(
Xicohtencatl-Cortes et al., 2007
). In EPEC, the
bfp
operon is found on pEAF
(
Sohel et al., 1996
;
Stone et al., 1996
), and of the 14 genes, all except for
bfpH
must be expressed for BFP expression and function (
Anantha et al., 2000
). The
operon begins with
bfpA
, which encodes the main pilus subunit precursor, pre-
bundlin, and ends with
bfpL
. Expression of the
bfp
operon is controlled by
perA
,
an AraC-family transcriptional activator (
Gómez-Duarte and Kaper, 1995
;
Tobe
et al., 1996
).
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