In cell biology, a biased random walk enables bacteria to search for food and flee from harm. Bacteria propel themselves with the aid of flagella in a process called chemotaxis, and a typical bacteria trajectory has many characteristics of a random walk. They move forward for a certain distance, then the course is abruptly altered by a process called tumbling. The average change of direction is about 60°.
The presence of a food supply gradient adds bias to the random walk and hence the phrase biased random walk. When the bacterium moves away from an attractant like a food source (sugar) the tumbling frequency increases (tumbling, negative chemotaxis). This makes it possible for the bacterium to change course and head back to the food source. The bacterium is also able to steer away from a repellent such as a toxin by reducing the tumbling frequency (running, positive chemotaxis). The bacterium is able to detect a chemical gradient not by monitoring a difference in concentration in head and tail but rather by a temporal sensing mechanism keeping track of concentration through time and space. The biased random walk in bacterium motility may be applied in robotic sensors that are able to find the source of an oil spill in oceans.
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