The Princess Bride had everything – good guys, bad guys,
rodents of unusual size, ex-professional wrestlers. Vizzini
was supposed to be brilliant, so why didn’t he cure his own
speech impediment? Inconceivable!
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.|
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.
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.
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.
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.
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.
So last week and above we see that some bacteria can generate an
electrical current in oxygen and sulfur. A new study shows that
altering magnets can turn magnetotactic bacteria, which might
then be like the logic gates or 0/1 switches of a computer. I think
someone should be looking into building a completely bacterial
computer, with bacteria supplying the power and the circuitry.
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.
Next week – not every flagellum is the same, so we need another name. Ever hear of an undulipodium?
Zhang R, Chen YR, Du HJ, Zhang WY, Pan HM, Xiao T, & Wu LF (2014). Characterization and phylogenetic identification of a species of spherical multicellular magnetotactic prokaryotes that produces both magnetite and greigite crystals. Research in microbiology PMID: 25086260
Brennan CA, Hunt JR, Kremer N, Krasity BC, Apicella MA, McFall-Ngai MJ, & Ruby EG (2014). A model symbiosis reveals a role for sheathed-flagellum rotation in the release of immunogenic lipopolysaccharide. eLife, 3 PMID: 24596150
Khalil, I., & Misra, S. (2014). Control Characteristics of Magnetotactic Bacteria: Magnetospirillum Magnetotacticum Strain MS-1 and Magnetospirillum Magneticum Strain AMB-1 IEEE Transactions on Magnetics, 50 (4), 1-11 DOI: 10.1109/TMAG.2013.2287495
For more information or classroom activities, see:
A great video of chemotaxis, a neutrophil chasing a bacterium. One using chemotaxis to find, the other using it try and escape.
Magnetotactic bacteria –
Bacterial flagellar chemotaxis –