The process by which bacteria move toward or away from chemical stimuli is called chemotaxis. The bacterium detects chemical concentration gradients and biases its movement toward attractants, such as nutrients, or away from repellents, such as harmful chemicals.
Many bacteria do not steer continuously like miniature swimmers. Escherichia coli, for example, alternates between relatively straight “runs” and brief “ tumbles” that change direction. If conditions improve during a run, the cell continues swimming longer; if conditions worsen, it tumbles sooner. This creates a biased random walk toward favorable conditions.
Chemotaxis begins when chemicals bind to receptors, often methyl-accepting chemotaxis proteins, on the cell surface. Signals pass through proteins including CheA and CheY to the flagellar motors, changing swimming behavior. This is different from phototaxis, which responds to light, and aerotaxis, which responds to oxygen. Quorum sensing is also distinct: it is chemical communication that coordinates group behavior, not directed movement itself.