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[Prev] | [Next]Typically microbes in aqueous environments continually move around looking for nutrients. Even microorganisms in the soil have uses and opportunities for movement. Sometimes this movement is random, but in other cases it is directed toward or away from something. As a rough guide, bacteria want to move towards food or energy sources and away from toxic compounds. In other words, bacteria are capable of showing simple behavior that depends upon various stimuli.
There are several classifications of tactic responses and the category is based upon the stimulus that the movement is responding to.
The purpose of chemotaxis should be fairly evident: attraction to nutrients or avoidance of damaging compounds. A cell distinguishes chemicals that fall into either of these classes with receptor molecules near their surfaces that can tell if specific chemicals are in the environment. Many of these receptors are also involved in transport of their target molecules. Now it actually is trickier than it sounds. A cell does not merely want to sense the presence of nutrient, but wants to move toward the highest concentration of it. It does this by an amazing process that is sort of a primitive memory, where it essentially asks if its receptors are binding more or less of the compound than they were a moment ago ("a moment ago" being about 200 milliseconds.). If they are binding more, the cell keeps swimming, but if they are binding less, it tumbles and then swims in a random direction. This tumbling is the direct result of E. coli's ability to reverse the rotation of its flagella.
Phototaxis is somewhat different, but again the cells move toward optimal levels of specific wavelengths of light. Rhodobacter sphaeroides (a photosynthetic microbe) performs phototaxis by a mechanism analogous to chemotaxis in E. coli, but there are important differences. R. sphaeroides contains only one polar flagella, which rotates during a run toward light. If, during a run, light conditions worsen, the flagella stop rotating (instead of reversing its rotation). The microbe instead depends upon Brownian movement in the environment to turn it in a new direction. R. sphaeroides is also capable of modulating the speed of rotation of the flagella in response to the environment, swimming faster when conditions are improving and slower if conditions are deteriorating.
Aerotaxis refers to the ability of some bacteria to be respond to the presence of oxygen. It is mechanistically generally similar to what has just been described and depends on levels of dissolved oxygen in the environment, but whether a bacterium swims toward it or away depends on the type of metabolism that it has.
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