Scientists have discovered how bacteria transport the tiny
hair-like strands, called pili, which cover their outer surface
from the inside of the cell, where they are assembled, to the
exterior.
Pili are a key target for a new generation of antibiotics, as
without them the bacteria are unable to group together and to stick
to human cells causing infection.
The scientists, from the Institute of Structural and Molecular
Biology (a joint institute between University College London (UCL)
and Birkbeck, University of London), have revealed the structural
and energetic process by which the bacteria transport the pili
across their outer membrane.
In the cystitis bacteria, pili enable bacteria to group together
and then to attach to the wall of the bladder. The bladder cells
then engulf the bacteria and this makes antibiotic treatment very
difficult. Once inside the bladder cells the bacteria can lie
dormant, making recurrent infections common. Scientists believe
that a new generation of antibiotics could be developed to disrupt
the process of pili biogenesis, making the condition easier to
treat.
In 2011, the scientists from the Institute of Structural and
Molecular Biology uncovered how pili biogenesis is initiated by the
protein FimD - an usher in the outer cell wall. FimD is responsible
for recruiting subunits, assembling them into a pilus, and
secreting the pilus as it is being formed. From this work, the team
was able to develop a model for the way the usher carry out all
these tasks.
The latest research, funded by the Medical Research Council,
provides experimental proofs for the model proposed before and
shows how, once the subunits have been assembled, the new pilus is
secreted from inside the FimD protein to the exterior, via a pore,
across the outer bacterial membrane.
The team has now revealed that as the pilus is assembled, the
first subunit (FimH) engages with the usher and undergoes
structural changes. This creates the necessary energy for this
subunit, which forms the tip of the pilus, to displace the plug
which is normally found inside the pore, and to pass through the
pore itself. As subsequent subunits of the pilus pass into the
pore, other structural changes in FimH prevent the pilus retreating
back through the pore.
The scientists were also able to reveal that within the usher's
pore there are specific binding sites for the tip and the
subsequent subunits of the pilus. These binding sites are 180
degrees apart within the barrel of the pore (facing each other); so
that the pilus is held in a central position as it emerges from the
pore, facilitating its exit.
Furthermore, they showed that as the pilus passes through the
pore it follows a rotational and translational path, enabling
subsequent subunits to continue being added within the usher as the
tip emerges, while the pilus is still held in a central
position.
Professor Gabriel Waksman said: "For the first time we have been
able to see the structural and energy pathways via which the FimD
usher protein facilitates the transport of the newly assembled
pilus across the outer membrane of the bacteria. This process is a
key target for the development of new antibiotics, as if biogenesis
of new pili can be disrupted the bacteria will be unable to attach
themselves to human cells and infection will be much less
likely.
"We have been working for a number of years to try and
understand the process of pili biogenesis and an understanding of
this process takes us a step closer to the development of new
antibiotics which will successfully treat cystitis - a common and
extremely painful condition, as well as other bacterial
infections."
The paper, Structural and energetic basis of folded-protein transport by the
FimD usher, is published in Nature.