Representation of the motions of slingshot bacteria. |
Bacteria
use various appendages to move across surfaces prior to forming
multicellular bacterial biofilms. Some species display a particularly
jerky form of movement known as “twitching” motility, which is made
possible by hairlike structures on their surface called type IV pili, or
TFP.
“TFP
act like Batman’s grappling hooks,” said Gerard Wong, a professor of
bioengineering and of chemistry and biochemistry at the UCLA Henry
Samueli School of Engineering and Applied Science and the California
NanoSystems Institute (CNSI) at UCLA. “These grappling hooks can extend
and bind to a surface and retract and pull the cell along.”
In
a study to be published online this week in Proceedings of the National
Academy of Sciences, Wong and his colleagues at UCLA Engineering
identify the complex sequence of movements that make up this twitching
motility in Pseudomonas aeruginosa, a biofilm-forming pathogen partly
responsible for the deadly infections seen in cystic fibrosis.
During
their observations, Wong and his team made a surprising discovery.
Using a high-speed camera and a novel two-point tracking algorithm, they
noticed that the bacteria had the unique ability to “slingshot” on
surfaces.
The
team found that linear translational pulls of constant velocity
alternated with velocity spikes that were 20 times faster but lasted
only milliseconds. This action would repeat over and over again.
“The
constant velocity is due to the pulling by multiple TFP; the velocity
spike is due to the release of a single TFP,” Wong said. “The release
action leads to a fast slingshot motion that actually turns the bacteria
efficiently by allowing it to over-steer.”
The
ability to turn and change direction is essential for bacteria to adapt
to continually changing surface conditions as they form biofilms. The
researchers found that the slingshot motion helped P. aeruginosa move
much more efficiently through the polysaccharides they secrete on
surfaces during biofilm formation, a phenomenon known as shear-thinning.
“If
you look at the surfaces the bacteria have to move on, they are usually
covered in goop. Bacterial cells secrete polysaccharides on surfaces,
which are kind of like molasses,” Wong said. “Because these
polysaccharides are long polymer molecules that can get entangled, these
are very viscous and can potentially impede movement. However, if you
move very fast in these polymer fluids, the viscosity becomes much lower
compared to when you’re moving slowly. The fluid will then seem more
like water than molasses. This kind of phenomenon is well known to
chemical engineers and physicists.”
Since
the twitching motion of bacteria with TFP depends of the physical
distributions of TFP on the surface of individual cells, Wong hopes that
the analysis of motility patterns may in the future enable new methods
for biometric “fingerprinting” of individual cells for single-cell
diagnostics.
“It
gives us the possibility of not just identifying species of bacteria
but the possibility of also identifying individual cells. Perhaps in the
future, we can look at a cell and try to find the same cell later on
the basis of how it moves,” he said.
The
study was funded by the National Institutes of Health and the National
Science Foundation. The lead authors are Fan Jin from the UCLA
Department of Bioengineering, the UCLA Department Chemistry and
Biochemistry, and the CNSI, and Jacinta C. Conrad of the department of
chemical and biomolecular engineering at the University of Houston.
