How do bacteria find the optimal concentration of oxygen?

Aerobic bacteria avoid the twin dangers of too little oxygen or too much oxygen by utilizing their electron transport system as the sensor for a positive behavioral response to oxygen (aerotaxis) and a different receptor for negative aerotaxis. Salmonella typhimurium swims up a gradient of oxygen until the terminal oxidase (cytochrome o) is saturated by oxygen. If the bacteria happen to swim too far up the gradient they are repelled by the high oxygen concentrations. The mechanism of positive aerotaxis is quite different from chemotaxis. Adaptation to most chemicals is dependent on methylation of a transducer protein, but adaptation to oxygen is independent of methylation.