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Surface Structures

©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

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.

Structure

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

Chemical Structure

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

Hook

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.

Flagellar synthesis

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.

Movement.

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.

Detection of motility

Indirect - looking for the presence of flagella

1. 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.

2. 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.

3. Electron microscope - The high magnification of the EM along with common staining practices make them easily visible.

Direct - looking for movement

1. Microscope. It is possible to watch living bacteria swim around using the phase microscope. In many cases this motility is due to flagella

2. 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.

Directed Motility

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

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 rotation

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.

Shown here is an animation of what a twiddle might look like.

Neutral conditions

In plain medium containing no attractant or repellent, the length of runs is random and the bacteria move about the solution aimlessly.

Shown here is an animation of a microbe moving in a neutral environment with no attractant or repellent.

Attractant

When cells are put in an environment containing an attractant such as glucose, they will move toward the source of the attractant. The microbes are sensing the change in concentration of the attractant, the concentration gradient, as they move through solution. If they are moving up the gradient to higher attractant concentrations, the length of the run will increase. If they are moving down the gradient, the length of the run will be much shorter. In this way, the bacteria eventually moves to the source of the attractant.

Shown here is an animation of a microbe moving in an environment containing a gradient of attractant.

Repellent

The behavior here is exactly the opposite of an attractant. When the compound is nasty, such as HCl (an acid), the microbe will shorten runs that go up the gradient and lengthen those that take it away from the repellent.

Shown here is an animation of a microbe moving in an environment containing a gradient of repellent.

Remember that cells do not detect absolute amounts of chemicals but detect a change in concentration. They detect the gradient by using a complex molecular mechanism. The attractant or repellent binds to receptors in the membrane and this signal is passed through the cell, eventually regulating the direction of flagellar rotation. After being exposed to a stimuli for a period of time, the cells will quit responding to it. This is actually a type of chemical memory. and has made bacteria the display chemotaxis model systems for understanding simple behavior.

Different modes of motion

Not all microorganisms get around by using bacterial flagella. Eucaryotic microbes will use flagella, but they are larger and more complex than bacterial flagella. Also, other bacteria use gliding motility that depends upon contact with a solid surface. Under the microscope it seems as if they are sliding along the surface. The exact mechanism for gliding motility is unknown, but one hypothesis suggests that little circular motors (similar to the basal body of flagella) are spinning underneath the microbe. The circular force causes the bacteria to glide across a surface in a manner similar to a floor buffer waxing a floor. A second idea is that gliding bacteria lay down a slime that they slide across. Both of these ideas have evidence that supports them and they may both be true for different bacteria. Microbes that glide can be difficult to work with, which points out an important thing to remember about science. We tend to learn about and emphasize systems that are easy to experiment on. This is understandable, but realize there are important fields of study that have not been investigated and are sometimes ignored because the systems available to work with them are difficult.

Also, remember that the above material explains how chemotaxis works in E.coli. It is different in other bacteria. Rhodobacter sphaeroides moves using a single polar flagella. It runs when the flagella rotates, but does not tumble. During chemotaxis, if conditions become unfavorable, flagellar rotation ceases and the microbe stops. During stops the microbe will slowly change direction due to brownian movement and then off it runs in a new direction. R. sphaeroides is also able to regulate the speed of rotation, moving faster toward an attractant or away from a repellent.

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