Attractants and repellents
Attractants and repellents are compounds that stimulate the directed movement of microorganisms , in particular bacteria , towards or away from the compound. The directed movement in response to the presence of the attractant or repellent compound is a feature of a bacterial behavior known as chemotaxis.
Various compounds can act as attractants. Overwhelmingly, these are nutrients for the bacterium. Attractant compounds include sugars, such as maltose, ribose, galactose, and amino acids such as L-aspartate and L-serine.
Similarly, various compounds will cause a bacterium to move away. Examples of repellents include metals that are damaging or lethal to a bacterium (e.g., cobalt, nickel), membrane-disruptive compounds such as indole, and weak acids, which can damage the integrity of the cell wall.
The presence and influence of attractants and repellents on the movement of bacteria has been known for over a century. In the 1880s experiments demonstrated that bacteria would move into capillary tubes filled with meat extract and away from capillaries filled with acids.
Now, the molecular underpinning for this behavior is better understood. The chemotaxis process has been particularly well-studied in the related Gram-negative bacteria Escherichia coli and Salmonella typhimurium.
These bacteria are capable of self-propelled movement, by virtue of whip-like structures called flagella. Movement consists typically of a random tumbling interspersed with a brief spurt of directed movement. During the latter the bacterium senses the environment for the presence of attractants or repellents. If an attractant is sensed, the bacterium will respond by exhibiting more of the directed movement, and the movement will over time be in the direction of the attractant. If the bacterium senses a repellent, then the periods of directed movement will move the bacterium away from the compound. Both of these phenomena require mechanisms in the bacterium that can sense the presence of the compounds and can compare the concentrations of the compounds over time.
The detection of attractants and repellents is accomplished by proteins that are part of the cytoplasmic, or inner, membrane of bacteria such as Escherichia coli and Salmonella typhymurium. For example, there are four proteins that span the inner membrane, from the side that contacts the cytoplasm to the side that contacts the periplasmic space. These proteins are collectively called the methyl-accepting chemotaxis proteins (MCPs). The MCPs can bind different attractant and repellent compounds to different regions on their surface. For example, on of the MCPs can bind the attractants aspartate and maltose and the repellents cobalt and nickel.
The binding of an incoming attractant or repellent molecule to a MCP causes the addition or removal of a phosphate group to another molecule that is linked to the MCP on the cytoplasm side. Both events generate a signal that is transmitted to other bacterial mechanisms by what is known as a cascade. One of the results of the cascade is the control of the rotation of the flagella, so as to propel the bacterium forward or to generate the random tumbling motion.
The cascade process is exceedingly complex, with at least 50 proteins known to be involved. The proteins are also involved in other sensory events, such as to pH , temperature, and other environmental stresses.
The memory of a bacterium for the presence of an attractant or repellent is governed by the reversible nature of the binding of the compounds to the bacterial sensor proteins. The binding of an attractant or a repellent is only for a short time. If the particular compound is abundant in the environment, another molecule of the attractant or repellent will bind very soon after the detachment of the first attractant or repellent from the sensor. However, if the concentration of the attractant or repellent is decreasing, then the period between when the sensor-binding site becomes unoccupied until the binding of the next molecule will increase. Thus, the bacterium will have a gauge as to whether its movement is carrying the cell towards or away from the detected compound. Then, depending on whether the compound is desirable or not, corrections in the movement of the bacterium can be made.