Understanding
bacteria motility would not only expand our understanding of their behavior,
but would also help us fight certain aggressive pathogens. However, the
question has gone unanswered because microbiologists have lacked the tools to
visualize bacterial filaments directly.
Lorenzo
Talà, a PhD student in the lab of Alexandre Persat at EPFL's Institutes of
Bioengineering and Global Health has developed
a microscopy method that can directly observe the structures many
bacteria use to crawl.
"Bacterial
surfaces are decorated with protein filaments involved in motility, adhesion,
signaling and pathogenicity, which ultimately rule how bacteria interact with
their environments" says Talà. "However, they are so small that
observing them in live cells is extremely complex. So we are left with little
knowledge of their dynamic activities."
But
do single bacteria orchestrate type IV pili motion to power their motility?
"In our studies of type IV pili and mechano-activation of virulence in Pseudomonas
aeruginosa, one technical paradox has been a source of frustration: pili, but
also fimbriae, flagella, and injection systems permanently extend outside
single cells, so why can't we directly visualize them?"
To
overcome this, the scientists explored an emerging microscopy method pioneered
by their collaborator Philipp Kukura at Oxford University. Using a technique
called interferometric scattering microscopy (iSCAT), they were able to see
these nanometers-wide filaments in live cells, without any chemical labels, at
high speed, and in three dimensions.
"iSCAT
represents a major technological advance in microbiology," says Persat.
"We recently described the visualization technique and received extensive
positive feedback among scientists across a variety of disciplines simply
because we could finally dynamically observe pili in live bacteria straight out
of culture."
To
understand the coordination of type IV pili movements, the scientists focused
on precisely timing the succession of surface attachment, retraction, and cell
body displacements using iSCAT. The approach revealed three key events that
lead to successful and energetically efficient movement across surfaces.
First,
contact of the pilus tip with the surface activates a molecular motor that
initiates retraction. Second, this retraction enhances the attachment of the
pilus to the surface, increasing the bacterium's displacement. Finally, a
second, stronger molecular motor enforces the bacterium's displacement under
high friction.
This
sequence shows that pili act as sensors, and reveals a new mechanism by which
bacteria interact with surfaces. It also reveals that bacteria use sensory
mechanisms to coordinate the dynamic motion of their motility machineries, in a
striking analogy to the way higher organisms, including humans, move their
limbs to generate displacement.
"The
human central nervous system processes mechanosensory signals to sequentially
engage motor components, thus triggering muscle contraction and resulting in
gait," explains Talà. "Our work shows that in the same manner,
bacteria use a sense of touch to sequentially engage molecular motors,
generating cycles of pili extension and retraction that result in a walk
pattern."
See:
Posted by Dr. Tim Sandle, Pharmaceutical Microbiology
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