 |
|
The
Giant Devil Ray, or mobula ray (Mobula
mobular) can reach
18
ft. (5.4 m) wide. It’s not so much that they fly or glide, they
just
breach the waves and look like they are trying to flap wings.
They
were almost fished to extinction in the 1970’s. Their meat
was
sold as scallops after they cut it out with a round cookie cutter!
|
The most common type of movement for bacteria is called run and tumble. Sounds a little like a
toddler learning to walk; however, the bacteria aren’t falling down, it’s more
like run and wander for them. The run is easy enough to explain; the
flagella we talked about last week spin and the bacteria swims forward in its
fluid environment like a little torpedo.
It’s not quite
that simple, but close. We explained last time that a flagellum is made of subunits of the flagellin protein and that these are joined together into a
hollow helix. The helix is most often left-handed (as you rise, the curve moves
to the left). So when these bacteria spin their flagella counterclockwise
(looking from behind the flagellum), the helix is pressed tight together and
spins efficiently – lots of forward movement. For those bacteria with right-handed
helices in their flagella, a clockwise spin is for forward movement, but this
is less common.
When the flagellum/flagella rotate the opposite direction,
you might think they would go backwards, but not so much. Many bacteria have more than
one flagellum and they work together when all spinning one for forward motion (more next week).
They bundle together like the trailing hair of a girl who is swimming forward
in a pool. But what happens when she stops or turns around quickly? Her hair
ends up in a tangled mess and she has to brush it out of her eyes – that is
unless she starts swimming again, then it trails behind in a bundle again.
 |
|
The
run of a bacterium uses flagella that are all rotating the same
direction
and work together. The tumble occurs when the turn
differently
and work against each other. On the bottom, the left
side
shows a random path with runs and tumbles, but the right
shows
how a bacterium can get closer to a food source when run
goes
up gradient but every tumble is completely random.
|
For a bacterium with a single flagellum, the reversal of
spin pushes the bacterium backwards, but then it runs into the flagellum and
all efficiency is gone. In a motor boat, the propeller is fixed a certain
distance from the back of the hull, so when it reverses direction, the movement
may be less efficient, but the boat doesn’t run into its own propeller. But
with a flagellum, the bacterium gets pulled right into the flagellum and
movement is hampered severely. The tumble begins.
Tumbling is just a random turning based on the various
places the flagella are inserted into the bacterial cell, the nature of the
flow of the fluid the bacterium is in, and the efficiency of the movement.
However, after a small tumble time, they will spin forward direction and the
bacterium will take off running forward again, probably in a new direction. The
purpose the run and tumble is to move toward something good (source of food) or
away from something bad (predator or chemical). More on this in a couple of
weeks.
Of course, there are exceptions. Some marine bacteria (those
that swim in salt water) have one flagellum and can reverse direction by
rotating their flagellum the opposite direction. This works for a while and
actually works better for reversing motion than having several flagella would.
However, a new study shows that they don’t reverse for long, they quickly
execute a trick called a flick.
Their flagellum flicks in one direction, turning them so that when they run
again, it will be in a new direction.
The researcher’s paper shows that this "reverse and flick" is a very efficient
way of turning. Some of these bacteria can move up gradients toward food faster
than bacteria that use the run and tumble method. "Reverse and flick" is a
good strategy, just like the “bend and snap maneuver from the movie Legally Blonde.
 |
|
The
axial filaments of spirochetes are really several flagella
that
lie in a ribbon. They work together to rotate under the
“skin”
of a bacterium, which causes a helical wave that
propels
the organism along. Scientists didn’t have this
mechanism
for a long time because when they prepared
bacteria
for electron micrograph, the flagella would pile up
on
one another and the coordinated rotation hypothesis just
wouldn’t
work that way. It was a preparation artifact that s
topped
our learning for a couple of decades.
|
According to a 2005 paper, these 7-11 flagella lie in a
ribbon that wraps around the cell body. By rotating counterclockwise, the
flagella put a torque into the cell body that makes it spin the opposite
direction, this drives the spirochete forward. See the image to the right and
this movie to get a better picture.
If most bacteria use flagella to move, you just know that
some have to be finding a different way. Twitching
is a kind of bacterial motility that doesn’t need flagella at all. Even though
I could probably come up with several movie references for twitching, I will
refrain. Twitching makes use of small appendages that project from bacteria
cells called pili (pilus is the
singular, it comes from Latin for hair).
We have talked about them before in terms of trading DNA back and forth in lateral gene transfer, but here that are used to move the bacteria along.
Pseudomonas
aerguinosa bacteria are famous for twitching, but a surface has to be
involved, it isn’t possible in a liquid medium only. The proteins in type IV
pili are coiled like a slinky. They stretch out, attach to a surface, and then
retract forcefully. This jerks the bacterium forward. This was discovered in
the very late 1990’s, but they didn’t know how they turned until 2011.
 |
|
Have
you ever had a twitch in your eyelid? You swear everyone
can
see it. It occurs because of a spasm in the palpebral
portion
of the occularis occuli muscle. That short fast
movement
looks a little like the twitch of bacteria, except
they
do it by snapping back a pilus instead of contracting
a
muscle.
|
Another kind of surface motility is called gliding. This type of motion is more of
a mystery than twitching ever was. There’s more than one way to glide. The
first example of gliding can really be considered elegant twitching. It uses
type IV pili that stretch out and then retract, but it is much smoother than the
jerky movement created when twitching.
Another type of gliding is used by some cytophagia (cell-eating) and flavobacterial
organisms. This movement might work a little like a conveyor belt, where
proteins attach to the surface and then move along the cell’s surface from
front to back. As the proteins are moved backwards, the cell moves forward. Many
show a helical track along the surface of the bacterium, so that as the
proteins dislocate toward the back, the cell goes both forward and rotates
around its long axis – efficient, but they may get dizzy. A 2014 minireview paper shows that very different bacteria use the same mechanism, but the
proteins and force for motility are different.
 |
|
Slimer
from Ghostbusters left slime where he had been, but I don’t
know
that he used it to push him along – he flew. Bacteria that use
slime
have to be on a surface. Is it just me, or does Slimer look a lot
like
the snot monsters from the Mucinex commercials? Bacterial
slime
is a little like snot, but is made of most sugars, not mucin proteins.
|
Finally, one of the fastest bacteria on surfaces is called Mycoplasma mobile. It may use a
mechanism of motility previously unseen and evolutionarily stunning. A 2005 paper showed that if you lyse the M.
mobile with a detergent, but provide the resulting fragments with the
proper ions, they will still move along a surface. This suggested that the
mechanism was ion gradient driven and confined to the membrane.
More recent studies (here and here) suggest that the protein
mechanism in the membrane might look very similar to the cytoskeleton of a
eukaryotic cell. This would be either an evidence of an endosymbiotic
origin of the cytoskeleton or that very different organisms had the same great
idea, called convergent evolution. Either way, it’s cool.
 |
|
Myxococcus gets around. When alone, he
forms a slime trail
that
actually pushes him forward. When he is with his buds,
he
might push out pili and retract them to pull himself
forward,
he might use a conveyor belt system to spin himself
along,
or he might use both. The signals that control which
he
uses still need to be worked out – anyone out there
feel
up to that task?
|
Several strains of bacteria together known as Myxococcus use different types of
gliding at different times. When M.
xanthus is with other bacteria of his kind, they move using something
called social gliding, which is of
the conveyor belt type OR the elegant twitching type. But when he’s alone, he
performs adventurous gliding, which
uses slime extrusion. Humans call this social climbing, but sliminess
is certainly involved in both.
Speaking of social motility - bacteria working with other
bacteria; this just happens to be our topic for next week.
For
more information or classroom activities, see:
A great site from Harvard University with movies of many types of bacterial motility:
http://www.rowland.harvard.edu/labs/bacteria/movies/
A great site from Harvard University with movies of many types of bacterial motility:
http://www.rowland.harvard.edu/labs/bacteria/movies/
Bacterial
motility -
Run
and tumble –
Pili
–
Gliding
–
http://www.molecularmovies.com/showcase/
No comments:
Post a Comment