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©2001 Timothy Paustian, University of Wisconsin-Madison
Surface structures originate outside the cell membrane, sometimes being attached to it, and extend into the environment. Important structures include flagella, pili, fimbriae, and glycocaylyx.
Flagella are responsible for motility in most bacteria. There is a loose correlation between cell shape the presence of flagella. Almost all spirillum, half of all rods, and rarely cocci are motile via flagella. You can wave your hands a little to justify why the coccus morphology and motility are mutually exclusive. A spherical shape would probably cause much more drag on the cell than a rod, so if you are going to be motile, be a streamlined rod.
Flagella can be thought of as little semi-rigid whips that are free at one end and attached to a cell at the other. The diameter of a flagellum is thin, 20 nm, and long with some having a length 10 times the diameter of cell. Due to their small diameter, flagella cannot be seen in the light microscope unless a special stain is applied. Bacteria can have one or more flagella arranged in clumps or spread all over the cell. The figure below demonstrates some of the more common arrangements.
|Polar or Monotrichous||Lophotrichous||Peritrichous|
Figure 1 - Flagellar Arrangements
Flagella are mostly composed of flagellin (a protein) that is bound in long chains and wraps around itself in a left handed helix. The number of units, the wavelength and diameter of a single helix of the flagella are determined by the protein subunits.Below is a picture of a common flagella in a Gram negative bacteria.
Figure 2 - The structure of a flagella in a G- bacteria
The hook and basal body of the flagella attach it to the cell. These are also proteins and their structure is different in G+ and G- bacteria.
If a flagellum is cut off it will regenerate until reaches a maximum length. As this occurs the growth is not from base, but from tip. The filament is hollow and subunits travel through the filament and self-assemble at the end.
The flagellum is a rigid structure and rotates like a propeller. Rings in the basal body rotate relative to each other causing the flagella to turn. The energy to drive the basal body is obtained from the proton motive force. How protons drive the rotation of the flagella is unclear.
How fast do bacterial cells move? They average 50 µm/sec, which is about 0.00015 kilometers/hr. This may seems slow but remember their tiny size. Table 1 below demonstrates a better comparison.
Table 1. Relative Speeds of Organisms
Detection of motility
Indirect - looking for the presence of flagella
Dyes. Flagella can be coated with dyes like pararosaline or basic fuchsin. The binding of the dye adds extra width to the structure and absorbs light, making them visible.
Antibody stains - These antibodies recognize flagellin. By attaching a fluorescent or colored dye to the antibody and using a special microscope, it is possible to detect the flagella.
Electron microscope - The high magnification of the EM along with common staining practices make them easily visible.
Direct - looking for movement
Microscope. It is possible to watch living bacteria swim around using the phase microscope. In many cases this motility is due to flagella
Motility medium. This is a semisolid medium that will hold non-motile bacteria in place, but motile microbes can swim through it. The presence of turbidity throughout the tube is a positive test for the presence of motility.
Why are bacteria motile?
Typically microbes that live in aqueous environments will continually move around looking for nutrients. Sometimes this movement is random, but in other cases it is directed toward or away from something. In other words, bacteria are capable of showing simple behavior that depends upon various stimuli.
There are several classifications of tatic responses and the catagory is based upon the stimulus that the movement is responding to.
Chemotaxis - towards or away from a chemical stimulus
Phototaxis - towards or away from light
Aerotaxis - towards or away from oxygen
Magnetotaxis - orientation in a magnetic field
Magnetotaxis what is that? One example is Aquaspirillum magnetotacticum, which has magnetosomes. These structures orient themselves in a magnetic field (The earth's magnetic field under natural conditions). The microbe uses this to determine which way is up and that helps it to find nutrients or adjust its depth in an aquatic environment. Other animals have magnetosomes; birds, dolphins, tuna, green turtles. In these cases they are used for navigation on long migrations.
Chemotaxis is accomplished by sensing the environment and adjusting the rotation of the flagella in response to stimuli. Before we get into this, I will point out that most of this work has been done on E. coli. Chemotaxis can behave differently in other microbes.
Flagella can rotate clockwise or counterclockwise. When flagella rotate counterclockwise this creates a force pushing on the bacteria. In the case of E. coli the peritrichous flagella bunch together and all push from one side. This causes the bacteria to move in a straight line, called a run. When flagella rotate clockwise, they all pull on the microbe. With all these forces pulling in different directions, it causes the bacteria to tumble or twiddle. When the twiddling is over, the bacteria will start out a new run in a completely random direction.
Shown here is an animation of what a run might look like.