We humans have a variety of modes of movement: walking, running, crawling, and numerous others! So too do our microbial friends. The movement of every motile microbial species can fall under one of the following descriptors—swimming, swarming, twitching, gliding, and floating. The difference between these is largely due to the presence (or absence) and performance of a filamented structure. The most notable of these—and indeed the structure most important to us—is that of the flagellum.
The flagellum is a fantastic feat of molecular machinery. Composed of more than 30 different proteins and a complex structure, it’s one of only two known rotational motors in biology (the second belonging to our good friend ATP synthase). The flagellum is what enables microbes like Magnetospirillum magneticum to swim through liquids, such as the water columns of shallow freshwater where this organism is typically found.
While the flagellum is made of numerous kinds of proteins, the visible filament (pointed out by the black arrows on the figure below) is only comprised of one—perhaps unsurprisingly—named flagellin.
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M. magneticum, the focus of this article and a truly fascinating species, is a type of microbe that swims. Let us first clarify what the differences in between each of these types of motility are so that we can better understand the marvel of how M. magneticum moves!
Twitching, gliding (More about it found here), and floating are all means of microbial movement that don’t use flagella. On the other hand, swarming is a motility that does use flagella. This motility is characterized by a group of the microbes working together to move across a surface. This means of transport requires both the flagella and the release of a surfactant to reduce surface tension as the microbes are hyper-flagellate. While each of these motilities are interesting in their own right (Hover over their names to learn more!), swimming is the one we are most interested in.
Swimming is a means of motility used by M. magneticum and must take place in liquid, as it does for humans. Unlike us however, microbial swimming requires the presence of flagellum. Each individual cell moves as a separate actor under their own power. The movement is caused by the rotation of the flagella, which propels the microbe. Naturally, the direction of rotation of the flagella affects the microbe’s movement and can be described as either clockwise (CW) or counterclockwise (CCW) rotation. Depending on rotational direction, cells can “run”—which moves the microbe towards its target—or “tumble”—which stops the forward motion and allows the microbe to change direction. Otherwise these cells would only move in one direction forever!
However, not every microbial species has their flagella arranged in the same manner. Motile organisms that rely on flagella to get around have evolved a variety of ways in which to use this microscopic motor. Flagellar arrangements can range from monotrichous (one polar flagellum), to amphitrichous (a single flagellum on each end), lophotrichous (tufts of flagella at one or both ends), or peritrichous (lots of flagella located all over the cell body). Just imagine the coordination that would be required if you had multiple pairs of legs located all over your body! In the case of M. magneticum, one polar flagellum is located on each end of the cell, making it an amphitrichous bacterium (See Figure 1 below). But, before we delve into exactly how this microbe manages to get around, we’ll take a quick detour to explore some of the unique characteristics of this bacterium.
While some might not see the attraction, M. magneticum happens to have quite the magnetic personality. No! Really! This microbe possesses specialized organelles, termed magnetosomes, that contain magnetic nanocrystals (See Figure 1 above). The magnetosomes align in a chain within the cell and provide an internal compass for the microbe. With this, M. magneticum is able to align itself with the earth’s magnetic fields. To learn about other microbes that also utilize magnetosomes and some of their applications, click here. While this internal compass is typically used to help orient M. magneticum in its search for favorable living conditions, it also made it a great candidate for understanding how amphitrichously flagellated bacteria get around with their bipolarly located flagella.
Twitching, gliding (More about it found here), and floating are all means of microbial movement that don’t use flagella. On the other hand, swarming is a motility that does use flagella. This motility is characterized by a group of the microbes working together to move across a surface. This means of transport requires both the flagella and the release of a surfactant to reduce surface tension as the microbes are hyper-flagellate. While each of these motilities are interesting in their own right (Hover over their names to learn more!), swimming is the one we are most interested in.
Swimming is a means of motility used by M. magneticum and must take place in liquid, as it does for humans. Unlike us however, microbial swimming requires the presence of flagellum. Each individual cell moves as a separate actor under their own power. The movement is caused by the rotation of the flagella, which propels the microbe. Naturally, the direction of rotation of the flagella affects the microbe’s movement and can be described as either clockwise (CW) or counterclockwise (CCW) rotation. Depending on rotational direction, cells can “run”—which moves the microbe towards its target—or “tumble”—which stops the forward motion and allows the microbe to change direction. Otherwise these cells would only move in one direction forever!
However, not every microbial species has their flagella arranged in the same manner. Motile organisms that rely on flagella to get around have evolved a variety of ways in which to use this microscopic motor. Flagellar arrangements can range from monotrichous (one polar flagellum), to amphitrichous (a single flagellum on each end), lophotrichous (tufts of flagella at one or both ends), or peritrichous (lots of flagella located all over the cell body). Just imagine the coordination that would be required if you had multiple pairs of legs located all over your body! In the case of M. magneticum, one polar flagellum is located on each end of the cell, making it an amphitrichous bacterium (See Figure 1 below). But, before we delve into exactly how this microbe manages to get around, we’ll take a quick detour to explore some of the unique characteristics of this bacterium.
While some might not see the attraction, M. magneticum happens to have quite the magnetic personality. No! Really! This microbe possesses specialized organelles, termed magnetosomes, that contain magnetic nanocrystals (See Figure 1 above). The magnetosomes align in a chain within the cell and provide an internal compass for the microbe. With this, M. magneticum is able to align itself with the earth’s magnetic fields. To learn about other microbes that also utilize magnetosomes and some of their applications, click here. While this internal compass is typically used to help orient M. magneticum in its search for favorable living conditions, it also made it a great candidate for understanding how amphitrichously flagellated bacteria get around with their bipolarly located flagella.
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| Figure 1. (A) Showing a Magnetospirillum magneticum cell with its two polar flagella marked by a black arrrow and its magnetosome chain marked by a white arrow. (B) Fluorescently labeled magnetosome protein. (C) Schematic showing cell body and flagellar rotation directions. Source. |
While most of what we know about how organisms use flagella to propel themselves is within the context of monotrichous and peritrichous bacteria, the orienting mechanisms of amphitrichous bacteria remained less well known-- up until rather recently that is. A 2015 paper by Murat et al., used our magnetic friend, M. magneticum, to explore how amphitrichous flagella are coordinated to steer swimming. The magnetic properties of M. magneticum simplified the process of observing and analyzing cell motility. By applying an external magnetic field, researchers were able to keep the organisms within one direction and limited to only performing one-dimensional runs. This made it easier to observe individual cells through video and photo imaging. In order to observe how the bacteria’s flagella rotated during swimming, the flagella were fluorescently labeled and observed.
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| Figure 2. Diagram showing the two characteristic flagellar patterns: parachutes (P) and tufts (T). Source. |
Researchers observed three movement types in M. magneticum: runs, tumbles, and reversals. When the organisms being observed performed runs, the researchers observed two different fluorescence patterns in the flagella. These patterns, termed “tuft” and “parachute” are aptly named (see Fig. 2 from the paper). Parachutes were seen on the leading end of the cell while tufts were seen on the lagging end. Further inspection revealed that the leading parachute flagellum rotated in the clockwise direction while the lagging tuft flagellum rotated in the counterclockwise direction. In order to change the direction of a run, the rotational direction of both flagella are changed. If both flagella rotate in the same direction however, a tumble is caused. These experimental results are the first to provide evidence for a model of movement in amphitrichous bacteria in which motility relies on asymmetrical flagellar rotation.
Videos of M. magneticum motility can be found here! (click on each video to download)
Motility, and thereby flagella are essential aspects to bacterial life. The mechanism by which amphitrichous cells coordinate flagellar rotation is poorly understood. Our previous levels of understanding had us solely examining motility on a species to species basis! Magnetospirillum magneticum AMB-1 are model organisms used to understand this style of life. In their research on M. magneticum, these scientists were able to reach a new level of understanding! They found that asymmetric flagellar rotation results in M. magneticum runs while symmetric rotation causes tumbling. Further examination of other microbes that have similar structure resulted in these researchers creating a possible motility paradigm for these spirillum-shaped bacteria with amphitrichous structure. They were able to show that the bipolar flagella are bidirectional, identify two different types of pauses in movement, see that there is likely variation in the torque of the flagellar motors, and most importantly, bring forth and substantiate a proposal for how these bacteria move!
Thanks to this experiment, we now can set aside the theory that one flagellar motor rotated at a time for motion, and instead can wholeheartedly embrace the model of simultaneous motion of the flagella. Because this mechanism of motility may be the most energy efficient, it is quite possible that a similar model of movement is used in morphologically similar microbes. This includes amphitrichously flagellated pathogens such as Campylobacter jejuni, one of the most common causes of food poisoning in the United States! This new model of motility opens the door for furthering our understanding of a whole host of microorganisms, not simply just our oh-so-attractive friend Magnetospirillum magneticum.
About the Authors:
Savannah Romeo '19
Savannah is a junior at Mount Holyoke College studying biology and statistics. When she isn’t watching copious amounts of baking shows, she’s pursuing her interests in public health and taking time to explore the fascinating world of microbes.
Lila Balakrishnan '19
Lila is a junior at Mount Holyoke College majoring in biology and politics. She spends her free time reading anything she can get her hands on—with a special place in her heart for fantasy books.
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