Bacterial Pathogenesis, Molecular Basis of
565
cause kidney infections (pyelonephritis).
Although P pili of many antigenic types
exist, they all appear to have the same type
of receptor (globobiose).
As mentioned above, surface molecules
that
are
important in adherence but
which do not form a piluslike structure
are often called the
nonpilus
adhesins
or
afmbrial adhesins
. In addition to the
Fmbrial adhesins, the uropathogenic
E.
coli
strains also carry adhesins that are
not
pili.
These
adhesins
include
the
aFmbrial adhesins Dr, A±AI, and A±AIII.
It is proposed that these molecules are
responsible for the tighter binding of
the bacterium to the host cell surface
subsequent
to
the
initial
binding
by
the pili. Very little is known about the
structure and functions of these types
of molecules, but it appears that some
of them may recognize host cell surface
proteins rather than carbohydrates. Many
pathogens maintain several alternative
means of adhering to cells that may be used
under speciFc environmental conditions
or to allow attachment to speciFc cell
types. Members of the genus
Yersinia
,
including
Y. enterocolitica
,
Y. pestis
,and
Y. pseudotuberculosis
,produceanumberof
adhesins that have also been implicated in
the invasion of host cells. These adhesins
(Inv, Ail, PsaA, and YadA) are extremely
interesting in that their expression is
often temperature-dependent and, in most
cases, they do not appear to use the
same receptors. Both the Inv and YadA
proteins are expressed at 37
Candbo
th
are capable of using
β
1-integrins as their
receptors. However, YadA can also bind
to laminin and Fbronectin. In addition,
the gene for Inv is chromosomally located,
whereas the gene for YadA resides on a
large plasmid carried by the organism.
In contrast, Ail can be expressed at
both 30
Cand3
7
C (albeit expression
is higher at 37
C) and although its
receptor has not been identiFed, it does
not appear to be the
β
1-integrins. PsaA
is interesting because it is expressed in
both
Y. pseudotuberculosis
and
Y. pestis
but not in
Y. enterocolitica
.Mo
reove
r
,in
contrast with all the other adhesins, it
appears to be a pilus. It is expressed
only at 37
Candon
l
ywh
enth
epHi
s
below 7.
Bordetella pertussis
,thecausativeagentof
whooping cough, provides an interesting
case for studies of the speciFcity of
adherence.
Bordetella pertussis
exhibits an
extraordinary preference for ciliated cells
of the respiratory tract. These organisms
produce a number of adhesins of which
the two best studied are the Flamentous
hemagglutinin (±ha) and pertussis toxin
(Ptx). Both adhesins lack the characteristic
ordered structure of a pilus. The receptors
for ±ha are the galactose residues of
sulfated glycolipids. Part of the preference
for the ciliated respiratory cells may be
due to the high content of a type of
sulfated glycolipid known as sulfatide in
the membranes of these cells. Mutants
in ±ha are still able to colonize the
host, indicating that other adhesins are
present.
The Ptx molecule is capable of func-
tioning both as a toxin and an adhesin.
Ptx is composed of six subunits termed
S1 to S5, all of which are present in sin-
gle copies except for S4, which is present
in two copies. During an infection, some
of the toxin molecules are secreted into
the surrounding environment, but the rest
remain attached to the bacterial cell sur-
face. These attached Ptx molecules can
now function as adhesins. The S2 and S3
subunits are responsible for binding to
receptors on the host cell. It is proposed
that these molecules act as adhesins when
they are left on the bacterial cell surface.
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