Type 1 and 5 secretion systems and associated toxins - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 505

Cloning and expression of the ehaG gene revealed that the EhaG TAA although
still mediating aggregation and biofilm formation, had different adhesion prop-
erties than those of UpaG. In contrast to UpaG, EhaG did not mediate adherence
to human bladder cells, but did promote binding to colorectal cell lines ( Totsika
et al., 2012 ). These functional properties correlated with the distinct tissue tro-
pism of EHEC and UPEC pathogens ( Totsika et al., 2012 ). Interestingly, the
UpaG/EhaG positional ortholog in Salmonella enterica , SadA, is also shown
to mediate adhesion to human intestinal cell lines. The SadA protein was also
found to elicit an IgG reaction in mice, and provide limited protection from
S. enterica challenge ( Raghunathan et al., 2011 ). Finally, the expression of
many T5SS genes, including upaG and ehaG , have been shown to be dependent
on the global regulator HNS ( Totsika et al., 2012 ).
The role of the type 5e protein, Intimin, in pathogenesis has been covered
extensively in Chapters 4 and 5 and thus has not been re-iterated here.
TYPE 1 AND 5 SECRETED PROTEINS AS PROSPECTS FOR
VACCINES
The T1SS is being used primarily for two types of biotechnological applica-
tions; mass production of extracellular chimeric polypeptides and live vac-
cine delivery. For mass production, it was found that by fusing proteins to the
C-terminus of HlyA, the whole protein was still recognized and secreted by
E. coli via the type 1 secretion pathway ( Blight and Holland, 1994; Gentschev
et al., 1996; Tzschaschel et al., 1996 ). This mechanism allows an easy method
to express and secrete fusion protein directly into the extracellular medium.
There seem to be very few limitations concerning the size or origin of the het-
erologous regions of the fusion proteins, with sizes ranging from 20 aa to above
1000 aa and sequences derived from prokaryotes as well as eukaryotes ( Catic
et al., 1999; Orr et al., 1999 ).
A highly important application of the T1SS however, is the presentation
of heterologous antigens in attenuated Gram-negative live vaccines. A bacte-
rial carrier strain can be used to secrete antigens for virtually any bacterial,
viral, and parasitic pathogens. A variety of antigens has been expressed and
secreted in attenuated bacterial carrier strains for use as vaccines against patho-
gens including Listeria monocytogenes ( Hess et al., 1996 ), Shigella ( Su et al.,
1992 ), Clostridium difficile ( Ryan et al., 1997 ), and the measles virus ( Spreng
et al., 2000 ). Protection against some of these pathogens has been observed
after vaccination with the recombinant live vaccines ( Hess et al., 1996; Ryan
et al., 1997 ). The T1SS has many advantages as a vehicle for antigen display
including: (1) the lack of size limitations for the protein; (2) its suitability for
the secretion of several antigens in a single carrier bacterium, allowing the con-
structions of multivalent vaccines ( Catic et al., 1999 ); and finally (3) its function
as a delivery system for immunocontraceptive vaccines and for co-expression
and co-delivery of active cytokines ( Hahn et al., 1998 ). Therefore, the use of