You don’t believe it now, but in the weeks ahead we’re going to discuss how bacterial motility, plant reproduction, intelligence, and the location of your heart are all related to whips and eyelashes. Sounds preposterous, but give me a few posts and a little leeway and you’ll be amazed.
The bacterial motor is called the flagellum, but it's so much more than just a way to get around, it’s often the means to saving their own lives. The word flagellum comes from the Latin word flagrum meaning whip, so you can see we are already starting to work on our challenge for these posts. Flagrum could also mean scourge, and this seems to be prophetic, since many flagella (the plural) we study have a hand in causing disease.
In typhoid fever, a potentially deadly disease that affects more than 20 million people each year, flagella are important not just for putting the bacterium, Salmonella enterica typhi, in the correct place to cause disease, but for attaching the bacterium to the gut wall and for invasion of the gut. A study in 1984 showed that even flagella that couldn’t move were still needed for S. typhi to produce disease. More about this in a couple of weeks.
A flagellum is very much like a boat propeller, it spins to produce force along its axis. This is possible because flagella aren’t perfectly straight. One of the main components of the flagellum is called the hook. This hook is located just outside the cell wall and lies in between the basal body and the filament. The basal body is the engine and is what attaches the flagellum to the cell, while the filament is the long whip like end that sticks out into the world.
The basal body attaches the filament and hook to the cell, and is made up of several rings. In Gram+ bacteria there are two basal body rings that anchor the flagella apparatus, the M ring which attaches to the membrane and the P ring which is anchored in the peptidoglycan layer. In Gram- bacteria, the basal body is longer and has more rings since it must anchor the flagella into the LPS (the L ring) and the M ring has a buddy in the inner membrane called the S ring. All these rings support the rod, which is turned by the rotor and then spins the hook and the filament.
The filament is pretty cool. It’s either a left- or right-handed helix of subunits of a protein called flagellin. The filament is a prescribed length in each bacterium, but we aren’t exactly sure how the length is regulated. Scientists know that it grows faster at first and then slows down, but if broken it will start to grow again at the faster pace.
The energy for the motion of the flagellum comes from the movement of ions across the membrane of the cell. We have seen before how protons (or other ions) being pumped out and then allowed to enter through a pore can create the force needed to do work. That’s how ATP is made, how the neural action potential works, and how photosynthesis proceeds. But here, the proton motive force is used to spin the hook and the filament, driving the bacterium forward.
The flagella spin one way to move forward, but when they spin the other way, the bacterium just sort of tumbles around. We’ll talk more about this next time. We’re just now starting to learn how the motor can go from spinning counter clockwise (forward motion for a left handed filament) to spinning clockwise in no time whatsoever and without slowing down. Nothing looks very different in the basal body, the hook or the filament, but the direction of spin is reversed.
Several studies have shown this change, and it is hypothesized that the change moves charged amino acids of FLiG around in relation to the cation gradient. By changing them, it changes the direction of the turning of the rotor (see the picture to the right). This might be akin to reversing the poles of a mag-lev train by flipping the electrical charge can make the train go the opposite direction.
Different bacteria have flagella that look similar but they have small differences. Nevertheless, it can be seen that this is a very complex machine for such a supposedly “primitive” domain of organisms. We have to remember that bacteria have been here the longest; they must be doing something right. There are over 40 genes that are required to build a flagellum, and they all fit together just so.
This complexity and order leads some people to declare that flagella couldn’t have evolved on their own. The concept is called irreducible complexity. People who support the idea of intelligent design (ID) say that some biologic components are so complex and have so many working parts that they could not arise through a series of mutations.
All the parts of a flagellum must be present for it to work (therefore they say it is irreducible) and must be assembled all at once which suggests it could not be random (complex in ID means improbably occurs by chance). Therefore, a flagellum could not have evolved over time and, ipso facto, it must have been designed as one unit by someone or something.
ID proponents haven’t always focused on the flagellum. They first talked of the blood coagulation cascade as irreducibly complex, but then it was shown that portions of the cascade were not necessary for function – whales don’t have factor XII and jawless fishes only use about half the proteins that vertebrates use. It was also shown how the cascade evolved over time.
But each time, the ideas of specified, irreducible and complex (must have all come together at once) have been refuted for each example.
For the bacterial flagellum, arguments against ID include the facts that different bacteria use different systems, although they are all variations on a theme. One exception is the Vibrio. They use two different kinds of flagella on the same cells, each needing its own genes. Likewise some bacteria don’t use protons for the gradients, they use Na+ ions. The bacterium Vibrio parahemolyticus is an exception in both cases.
It uses a single flagellum at its end (polar) to swim in liquid water, but many flagella all around its cell when in something thicker. The polar flagellum uses Na+ ions to drive the rotor, while the lateral ones use protons. The genes are different for each flagellar type and mutations in one don’t hurt the other.
Spirochetes don’t even have flagella that protrude from the cell, they’re located between the inner and out membranes (endoflagella). This is a different system and again argues against irreducible complexity in flagella, unless different systems were designed differently. More about spirochete motion next week.
Please read more about ID and decide for yourself if it holds up to the tenets of science - that something that is true must be observable, repeatable, and able to be refuted if incorrect. Irreducible complexity is refutable, and has been for every example proffered by ID. But the conclusion that ID draws – that a designer must be involved, is a belief not a hypothesis – you can’t refute a belief, it doesn’t rely on observable evidence, therefore ID is not science. It doesn't make it wrong - it just makes it faith, not science.
Next week, let’s look at the different ways flagella help bacteria move, and some exceptions in bacterial motility.
For more information or classroom activities, see:
Bacterial flagella –
You must be careful to vet the source of material on flagella, much “science” is actually put out by Intelligent Design proponents, masking it as science.
Intelligent design –
Typhoid fever and Typhoid Mary –
Vibrio bacteria -