Many microorganisms possess the ability to move toward a chemical environment favorable for growth. They will move toward a region that is rich in nutrients and other growth factors and away from chemical irritants that might damage them. Among the organisms that display this chemotactic behavior, none is simpler than bacteria.
Bacteria are single-celled prokaryotic microorganisms, which means that their deoxyribonucleic acid (DNA) is not contained within a well-defined nucleus surrounded by a nuclear membrane, as in eukaryotic (plant and animal) cells.
Prokaryotes lack many of the cellular structures associated with more complex eukaryotic cells; nevertheless, many species of bacteria are capable of sensing chemicals in their environment and responding by movement.
Bacteria capable of movement are called motile bacteria. Not all bacteria are motile, but most species possess some form of motility. Although there are three different ways in which bacteria can move, the most common means is by long, whiplike structures called flagella.
Bacterial flagella are attached to cell surfaces and rotate like propellers to push the cells forward. A bacterial cell must overcome much resistance from the water through which it swims. In spite of this, some bacteria can move at a velocity of almost 90 micrometers per second, equivalent to more than one hundred bacterial cell lengths per second.
A flagellum is composed of three major structural components: the filament, the hook, and the basal body. The filament is a hollow cylinder composed of a protein called flagellin.
A single filament contains several thousand spherically shaped flagellin molecules bound in a spiral pattern, forming a long, thin cylinder. A typical filament is between 15 and 20 micrometers long but only 0.02 micrometer thick.
The filament is attached to the cell by means of the hook and basal body. The hook is an L-shaped structure composed of protein and slightly wider than the filament. One end of the hook is connected to the filament, and the other end is attached to the basal body.
The basal body, also known as the rotor, consists of a set of protein rings embedded in the cell wall and plasma membrane. Inside these rings is a central rod attached to the hook. The central rod of the basal body rotates inside the rings, much like the shaft of a motor. As it rotates, it causes the hook and the filament to turn.
Bacteria in Motion
While they are moving, bacteria change direction by reversing the rotation of their flagella. As a bacterium swims forward in a straight line, its flagella spin in a counter clockwise direction.
Because of their structure, the flagella twist together when they rotate counter clockwise and act cooperatively to push the cell forward. The forward movement is referred to as a run.
Every few seconds, a chemical change in the basal body of each flagellum causes it to reverse its spin from counter clockwise to clockwise. When the flagella spin clockwise, they fly apart and can no longer work together to move the cell forward.
The cell stops and tumbles randomly until the flagella reverse again, returning to counterclockwise spin and a forward run. This type of movement, in which the cell swims forward for a short distance and then randomly changes its direction, is called run and tumble movement.
Certain eukaryotic microorganisms, such as Euglena and some other protozoa, are also motile by means of flagella. The structure and activity of eukaryotic flagella are, however, completely different from those of bacteria.
Eukaryotic flagella are composed of protein fibers called microtubules, which move back and forth in a wave like fashion to achieve movement. The rotation of bacterial flagella and the run and tumble movement they produce are unique to bacteria.
Attractants and Repellants
Bacteria respond by chemotaxis to two broad classes of substances, attractants and repellants. They move toward high concentrations of attractants (positive chemotaxis) and away from high concentrations of repellants (negative chemotaxis).
|Attractants and Repellants|
Attractants are most often nutrients and growth factors, such as monosaccharides (simple sugars), amino acids (the building blocks of protein), and certain vitamins required for bacterial metabolism. Repellants include waste products given off by the bacteria as well as other toxic substances found in the environment.
Bacteria respond to attractants and repellants by altering the time between tumbles in their run and tumble movement. When a bacterial cell detects an attractant, the time between tumbles and the time of the runs increase.
As long as the cell is moving toward a higher concentration of attractant, its runs will be longer. The opposite effect occurs when a cell encounters a repellant.
A repellant causes the time between tumbles to decrease, resulting in shorter runs as the cell changes direction more frequently while trying to avoid the repellant. The net result is that the cell tends to move toward a lower concentration of the repellant.
Bacteria recognize attractants and repellants through specialized proteins called chemotactic receptors, also called methyl-accepting chemotactic proteins (MCPs), which are embedded in their plasma membranes just inside the cell wall.
Biologists have identified roughly twenty different receptors for attractants and some ten for repellants. Each receptor protein is believed to respond to only a single type of attractant or repellant.
When an attractant molecule binds to its chemotactic receptor, two separate events occur. First, there is a rapid activation of the receptor. The attractant molecule binds to a special site on the receptor protein to form an activated receptor.
This binding is not permanent, however, so a cell must remain in an area with attractant molecules for its receptors to remain activated. The activated receptor sends a chemical signal to the basal bodies of flagella, which causes them to spin in a counter clockwise direction, producing continuous swimming in one direction.
At the same time, there is adaptation of the activated receptors to the attractant. Adaptation is important because it keeps the cell from swimming too long in one direction.
It is accomplished by methylation of the receptors, a process in which methyl groups are attached to the protein by an enzyme in the cell. (A methyl group consists of an atom of carbon attached to three atoms of hydrogen.) Methylated receptors do not stimulate the basal bodies for counter clockwise rotation as effectively as nonmethylated receptors.
After a cell has been in the presence of an attractant for a short while, its receptors adapt to the attractant, and it returns to the original pattern of run and tumble movement. Adaptation is reversed by demethylation, the removal of methyl groups from the receptor by a separate enzyme.
Together, the balance between methylation and demethylation makes the receptors very sensitive to small changes in attractant concentration, so that cells remain in the region with the greatest concentration of attractant.
The action of repellants appears to be similar to that of attractants. Repellant molecules bind to sites on their chemotactic receptors, activating the receptors.
The activated receptors signal the flagella to spin clockwise instead of counterclockwise, causing the cell to tumble and to change direction. Repellant receptors also adapt through methylation and demethylation, much like attractant receptors.
It is not entirely understood how an activated chemotactic receptor can signal flagella to rotate. Four different proteins inside the bacterial cell have been identified as a possible link between the chemotactic receptors and the basal bodies of flagella.
These proteins are believed to regulate flagellar rotation using a process called phosphorylation. Phosphorylation, the attachment of phosphate molecules to a protein, is used in all types of cells as a kind of “on and off” switch to regulate protein activity.