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The word that comes to our minds when bacteria take the stage is “chaos.”
Each individual may be so small as to feel insignificant, but the combined presence of
all of those tiny shapes in so many different states of excitement can be difficult to make
sense of.
In fact, in the 18th century, Carl von Linné grouped the microbes he observed together
and named them “Chaos.”
The label didn’t last in part because it wasn’t exactly specific, but the feeling
endures.
Even when the movement seems more ordered, it’s baffling to watch.
Like these little wiggles that seem so snake-like except for the unsettling vibration that’s
added on.
Or you might see a mass of bacteria collectively spinning in place like a very odd, microscopic
flash mob.
Or you could throw them all together on the same slide and end up with what looks like
several different dance parties at once.
But while the movements may be a bit much to take in when witnessed all at once, the
source of that movement is almost always the product of the same masterpiece of bacterial
ingenuity: the bacterial flagella.
We’ve been wanting to talk about bacterial flagella for a while, but it’s been tough
because well they’re just small and they’re difficult to make out in our footage.
Fortunately, with a lot of patience and magnification, we have finally been able to spot them for
the first time in our microcosmos journey.
But we have to zoom in a bit more on this clip, and you will need to ignore the rounder
protists buzzing around.
Instead, focus on the rod-shaped bacteria, and you will see these faint lines coming
off of them.
Those are the flagella.
More specifically, each strand you see is a filament, which is anchored to a joint called
the hook, which then connects to a motor enclosed in the bacteria itself.
Words like “hook” and “motor” imply something made from human hands, with metal
bits of machinery and fuel.
But biology builds with its own materials, and the flagella is a complex assembly made
out of proteins that move like gears.
And it does function very much like one of our motors, with a stator and a rotor and
gears that connect between them.
But, of course, both arose independently because if you want something to rotate, there is
a best way to do it, and both nature and humans, in this case, came up with the same solution.
Though, of course for bacteria the movement of the motor is powered by an electrochemical
potential created by pumping ions across the cell membrane instead of by an internal combustion
engine.
Now if you’ve been on this journey with us for a while, you might be wondering, “Why
are you talking about flagella like it’s a new thing?”
We’ve talked about tons of flagellates before, and even explained how they move.
But there is a difference here.
Those flagellates are eukaryotes.
And while the name is the same, the actual appendage is definitely not.
The bacterial flagella is a hollow tube that sticks out of the cell and rotates as dictated
by its motor.
It actually spins around.
The eukaryotic flagella is a bundle of microtubules surrounded by cell membrane and it does not
rotate, it just flips back and forth to whip around to move the cell.
And unlike the ion gradients used to power bacterial flagella, eukaryotes rely on the
energy-storing molecule ATP to fuel theirs.
So, why do these two different assemblies have the same name?
Well, probably simply because when we were originally looking at them, we didn’t know
any of this.
Understanding the biochemistry of these marvelous machines has been a very long, involved, careful
process involving thousands of scientists over decades.
One of the original tests to see how different bacterial flagella were from eukaryotic flagella
actually involved basically taping the bacterial flagella down.
And they found that when they did that, instead of flopping back and forth, the bacteria themselves
spun around and around.
The movement of bacteria stood out to early observers for the same reason it stands out
to us: because it is so weird.
There’s a mix of sporadic movements and directed movements.
But even with the significant lack of grace, there is something coordinated underlying
this movement.
It comes down to what direction the bacteria’s flagella are rotating.
When the motor turns the flagella clockwise, the filaments form a supercoil that bundles
together and that drives the bacteria straight.
But when the flagella is turned counter-clockwise, the bundle comes undone, and each individual
flagella moves on its own.
And that creates disarray, causing the bacteria to tumble randomly around the microcosmos.
In our world, stumbling around randomly might seem like a bit of a liability.
But for bacteria, this is an evolutionarily planned affair, helping the bacteria find
food and light.
Tumbling around lets the bacteria sample the world around them, looking for chemical scents
or hints of light that might point them towards the resources they need to survive.
And when they catch those signals, whether it’s a ray of light or a chemical gradient,
the flagella turn clockwise to set the bacteria on a straight course towards their next home.
So while at first glance it seems messy and even a little broken, the bacterial flagella,
it turns out, is one of the most majestic, useful, and powerful structures in biology.
Barely visible, but in our opinion, completely deserving of a fan club all its own.
Thank you for coming on this journey with us as we explore the unseen world that surrounds
us.
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