Chase The Good, Evade The Bad

Proximity is a good relative indicator of danger or benefit. As Vizzini said to Wesley in The Princess Bride, “As a student you must have learned that man is mortal and you would therefore put the poison as far from you as possible.” We tend to move toward things we need or want, and away from those things that could harm us – except for doughnuts of course.

A couple of weeks ago we started to talk about flagellar movement and the how a bacterium will “run” up a positive gradient or “down” a negative gradient. More detail will show us how amazing this chemotaxis (chemo = chemical, and taxis = arrangement) is.

The “run” in run and tumble movement is in a particular direction, while the tumble is a mess, just turning randomly before the run continues in another direction. What directs a run or a tumble? Well, they’re either running toward or running away from something.

There are receptor proteins on the surface of bacteria that sense different things. Some sense food; if food is to the left, receptors on the left will start to pick up more signals. As long the concentration keeps going up, the cell is directed to continue a run (positive chemotaxis). If the concentration starts to decrease (less signal for receptors), then a tumble is in order.

Random walking by run and tumble in bacteria.

Since the tumble is a random turn, the result doesn’t necessarily turn the bacterium toward food. If the concentration doesn’t start to increase as the next run starts, another tumble will commence and maybe then the organism will be faced the right direction (see animation). This works for twitches, glides, and rolls as well, and is particularly effective even if part of it is random.

Chemotaxis works the other direction as well. If a negative chemical is sensed, such as a predator or toxin, a run will continue as long as the concentration of the chemical keeps going down (negative chemotaxis). If the concentration stays the same or increases, a tumble will hopefully reorient the direction of movement down the gradient.

Remember that the movements for runs and tumbles are controlled by the flagella.  Not surprisingly, there are several different flagellar possibilities. Having one flagella is called monotrichous (mono = one, and trichous = hair), it’s usually at the long end of a bacterium.

A new paper has started to describe the symbiotic relationship
between the bobtail squid and Vibrio fischeri. The bacterium is

bioluminescent, and lights up the squid when it is in the

moonlight so it doesn’t cast a shadow from below (predators

would find it that way). It turns out that flagella of the bacteria

give off LPS a toxin, and the concentration tells the squid when

to alter the biochemistry of its light organ to accommodate the

needs of the bacteria. They work together to keep them both alive.

For example, many Vibrio organisms are monotrichous. They have one flagellum located on one end of their cell body, and it propels tem forward or in a tumble. One organism, Vibrio cholerae, is especially important to humans as it causes the disease cholera. This organism has a sheathed flagella (cell membrane covers the flagellin protein polymer on the outside). It has been hard to study this since unsheathed mutants are nonfunctional. See the caption at right for more.

Lophotrichous bacteria (lopho = crested or tufted) have tufts of multiple flagella at one (polar lophotrichous) or both ends of the organism. Spirillum volutans is lophotrichous - but not always. When it divides, each of the progeny has just one tuft of flagella, since each daughter gets one end of the parent. As they grow longer and older, they develop the second tuft of flagella at the opposite end.

           

S. volutans was first described in 1900's. The unusually large

flagella made them visible by light microscopy, the only type they

had at the time. Most other flagella had to wait for electron

microscopy to be discovered. S. volutans is a spirillum, shaped sort

of like a spirochete, and the flagella make the body spin in the

opposite direction, just like the spirochetes. But the spirochete has

the flagella on the inside, and the spirillum has them on the outside.

I wonder if one evolved from the other.

The question then is how S. volutans regulates movement with a tuft at each end. An older study showed that there is a head type tuft and a tail tuft in terms of sensing chemicals. When the tufts reverse their rotation, the tail tuft becomes the head tuft. There are chemicals that can make each tuft rotate as the head, and then the organism doesn’t go anywhere. This could become important for stopping disease development.

If a bacterium has one flagellum at each end it is considered amphitrichous (amphi = both). A good example is Campylobacter jejuni, the causative organism of the most common type of gastroenteritis (diarrhea). C. jejuni causes more disease each year than Shigella and salmonella combined, about 3 million cases – mostly from poorly cooked chicken.

           

A 2014 study on C. jejuni flagella show that it has necessary genes that are not found in other types of bacteria. Campylobacter flagella are some of the most complex and the motility they control is very important for pathogenesis. This flagellar system is just another example of how flagella can’t be seen as evidence for intelligent design.

           

Peritrichous (peri = around) bacteria are hippies. They have flagella that stick out in all directions; no sense of order or grooming. The quintessential peritrichous organism is E. coli. All the flagella turn the same direction in a run, but when just one or a few switch direction, they start a tumble. Since these organisms sense chemicals from all directions, they switch from runs to tumbles quicker and more often. As a result, peritrichous organisms are often faster in both + and – chemotaxis.

Selenomonad bacteria are bean shaped, with a long axis. But their

tuft of flagella is located on the long side, not on an end. So why

do they travel along their long axis? It might have something to

do with the degree of turn in their hook, or the curve of the

bacterial cell.

Notice that we've been talking about bacteria that have a long axis and a short axis. Their flagella are usually on their end(s). But there are exceptions. Selenomonad bacteria are polar lophotrichous, but the flagella aren’t on a long end. It’s weird, because they still move along their long axis. You need to figure out how they do that.

And what about the cocci? A coccus type microorganism is round (coccus = berry in Greek). Most cocci are immotile, they get moved around instead of moving around. But it hasn’t hurt them, as cocci are found everywhere the other shaped bacteria are found.

Being round may have something to do with their immotility. Round objects aren’t best designed for movement in a single direction. Think about it, almost all animals are motile (except some sponges and the Tribbles on Star Trek), but have you ever seen a spherical animal?

Things that are longer than wide are usually best equipped for linear movement. And if you aren’t going to move linearly (up or down a gradient), what’s the point of moving at all? Therefore, most cocci are flagella-less. Fortunately for us, there are exceptions to the exceptions. Some cocci do have flagella and are motile. Often, the flagellated cocci are polar lophotrichous - like a bald guy with a ponytail.

I was surprised to find that the term “coccus” doesn’t just apply to bacteria, archaea can be coccal as well. This may not seem like a big deal, but remember that archaea and bacteria are as divergent from one another as we are from bacteria. The point is that “coccus” is just a description of a shape, it doesn’t have to mean bacteria. Coccolithophores are eukaryotic phytopklankton, and the genus “coccus” plants are berry-forming vines or shrubs.

On the top is an electron microscopic image of the magnetosome

chain inside a magnetotactic bacterium. See how they line up along

a field line? The bottom cartoon is one hypothesis of why they

developed this skill. Perhaps they can find the right concentration of

oxygen to sulfur by traveling just along the field line, not in three

dimensions. This is sort of like the electrical cable bacteria we talked

about last week.

Pyrococcus furiosus (rushing fireball) is a 

lophotrichous archaea with up to 50 flagella. They swim very fast when in their optimum temperature water, around 100˚C, hence their name. A 2006 paper showed that the flagella aren’t just for swimming, but also for cell-cell adhesion and adhering to surfaces, but more about this in the future.

In terms of the flagellated cocci, the most interesting exceptions are the magnetotactic cocci. Magnetotactic bacteria come in many shapes and sizes, and examples can be found in many different bacterial family trees.

What these differently shaped magnetotactic bacteria have in common is that they contain tiny magnetic organelles (yes, bacteria can have organelles, see this post). There are basically two types of magnetic organelles, based on what metal they contain, but both are generated by the bacterium sequestering the metal and then storing it in a granule.

Because they contain magnets, magnetotactic bacteria line up along the magnetic field lines of the Earth. This was noticed as early as 1963 when an Italian scientist studying some bacteria on slides noticed that certain types of them always pointed north/south.

Since we're talking about cocci at the moment, you may ask how something that is spherical can line up in a direction. Well, some of them are flagellated, so you can see a direction, some of them string together to form streptococci (strepto = line) along a magnetic line, and some that don’t attach to each other will still line up by the hundreds according to magnetic lines introduced by a strong, close magnet.

A recent study has found what might be the first peritrichous coccus, and it's magentotactic as well. This paper refers to them as MMP – multicellular magnetotactic prokaryotes. These particular microorganism are always found in strings of a dozen to three dozen and have flagella sticking out on all sides.

Also a novelty, these new bacteria are the first magnetotactic bacteria known to have both types of magnetic granules; all others have one type or the other. The question - why have either type? What good does it do a bacterium to be aligned along the magnetic fields of the planet?

All the known magnetotactic bacteria, including all the coccal examples, are flagellated; therefore, it must be important for them to be motile. What’s the point of lining up with magnetic field lines if you just sit there, it should be involved in helping you get somewhere faster or better or putting you in a position to take advantage of something - so they’re all flagellated. The current hypothesis is that lining up with the field takes one plane of movement decision away from them, so they can move quickly toward food or oxygen. Sounds plausible.