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by analysis of a series of single, double, and triple mutant EPEC phenotypes
(
Cleary et al., 2004
). While wild-type EPEC adhered robustly and exhibited
LA, bacteria lacking BFP adhered poorly to host cells but could form intimate
attachments, provided the EPEC strain still expressed both a functional T3S
system and intimin.
In ETEC, the Longus pilus is important for aggregation. However, unlike
pilus mutants in EPEC, ETEC mutants deficient for
lngA
expression can still
adhere to human cell lines in vitro (
Mazariego-Espinosa et al., 2010
). ETEC
strains expressing CFA/III can colonize infant mice and rabbits (
Honda et al.,
1984
), human enterocytes (
Knutton et al., 1989
) and human carcinoma cells
(
Taniguchi et al., 2001
). CFA/III expression is also associated with aggregation
(
Kolappan et al., 2012
). However, CFA/III is one of numerous adhesins used by
ETEC and its role in virulence is unknown.
IMMUNE RESPONSES
At present, very little has been published about the potential immune response
to components in the inner and outer membrane assembly complexes from
any bacterial T2S or T4P system or their potential as vaccine candidates. As
the components of the IM assembly such as GspE or the prepilin peptidase
are not surface expressed in bacteria they would likely make poor vaccination
targets even if they were highly immunogenic. Mice immunized against the
C-terminus of PilQ, the secretin of the T4P system of
Neisseria meningitidis,
generated serum bacteriocidal activity against homologous and heterologous
strains (
Haghi et al., 2012
), but similar studies of
E. coli
secretins are lack-
ing. Rather than focusing on the various components, most research focuses
on the immunogenicity and vaccine efficacy of the proteins like LT secreted
via the T2S pathway (
Frech et al., 2008
;
Norton et al., 2011
). The immuno-
genicity of each product differs according to the protein in question, but at
least in the case of LT, immunizing volunteers with the exoprotein alone was
enough to provide protection against travelers' diarrhea (
Frech et al., 2008
).
The main pilus subunit of EPEC, bundlin, is immunogenic (
Loureiro
et al., 1998
;
Fernandes et al., 2007
) and antibodies against bundlin are pro-
duced in the course of an infection (
Donnenberg et al., 1998
;
Parissi-Crivelli
et al., 2000
). There is evidence from volunteer studies of antibody responses
against bundlin (
Fernandes et al., 2007
), but whether these responses are
protective is unknown. Bundlin has been expressed in an attenuated
Salmo-
nella enterica
serovar Typhimurium strain, but vaccine evaluation is difficult
without a relevant animal model of infection (
Schriefer et al., 1999
). It is
also possible that a bundlin-deficient mutant could be used as the basis for a
live attenuated vaccine candidate as in the case of
V. cholerae
(
Tacket et al.,
1998
). However,
bfpA
sequences vary. EPEC strains can be broken down into
two broad categories of
bfpA
sequence similarity (
Blank et al., 2000, 2003
;
Lacher et al., 2007
). All alleles are at least 80% identical, with the alpha
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