Bacteria Can Really Get Around

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!

How many different ways can humans move about? Walk, crawl, run, hop, swim, dance - you could say walk on hands or do the worm but I don’t think they count as normal modes of locomotion. Birds can fly, walk, or swim. Fish can swim and at least two think they can fly – flying fish and mobula rays. But the winners are……bacteria, again. They basically move forward, backward or turn, but they have several unique ways of accomplishing this.

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 quitethat 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.

This is similar to the flagellar movement, except that with bacteria the flagella are the source of the movement, not just passive followers. When they switch from forward direction spinning, the bundles fall apart and they each start pushing in a different direction. This is why the bacterium tumbles, it just jerks around turning in random directions.

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.

Spirochete bacteria use flagella to move as well, but they use them differently. We talked about this a bit last week. Spirochetes have internal flagella (called endoflagella) that run the length of their corkscrew shape in their periplasm (between inner and outer membranes).

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. Twitchingis 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.

A 2011 PNAS paper showed that they slingshot themselves. Some pili stretch out and attach. Others stretch out in another direction and then instead of retracting to pull the bacterium in that direction, they release at the tip. This shoots the organism in the other direction. It’s the moral equivalent of a tumble, just not using flagella.

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.

In a third form of gliding, the bacterium produces a slime that it then travels over, sort of like a snail or slug, maybe more like Slimer in Ghostbusters. In this form of gliding on a surface, a mix of polysaccharides is secreted from pores in the cell wall and membranes of the bacterium. The force of the release in one direction pushes the cell in the other direction. Think of it as very slow motion rocket propulsion.

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?

All these mechanisms just go to prove that bacteria have more ways of moving than we could ever dream up on our own. They have propellers, finger proteins to pull them along, conveyor belts, cytoskeletons, and even snot rockets. It must be important to get from on place to another if they have developed so many mechanisms. Some even combine their modes of transportation.

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.