Aerotaxis and other energy-sensing behavior in bacteria

Energy taxis is widespread in motile bacteria and in some species is the on ly known behavioral response. The bacteria monitor their cellular energy le vels and respond to a decrease in energy by swimming to a microenvironment that reenergizes the cells. This is in contrast to classical Escherichia co li chemotaxis in which sensing of stimuli is independent of cellular metabo lism. Energy taxis encompasses aerotaxis (taxis to oxygen), phototaxis, red ox taxis, taxis to alternative electron accepters, and chemotaxis to a carb on source. All of these responses share a common signal transduction pathwa y. An environmental stimulus, such as oxygen concentration or light intensi ty, modulates the flow of reducing equivalents through the electron transpo rt system. A transducer senses the change in electron transport, or possibl y a related parameter such as proton motive force, and initiates a signal t hat alters the direction of swimming. The Aer and Tsr proteins in E. coli a re newly recognized transducers for energy taxis. Aer is homologous to E. c oli chemoreceptors but unique in having a PAS domain and a flavin-adenine d inucleotide cofactor that is postulated to interact with a component of the electron transport system. PAS domains are energy-sensing modules that are found in proteins from archaea to humans. Tsr, the serine chemoreceptor, i s an independent transducer for energy taxis, but its sensory mechanism is unknown. Energy taxis has a significant ecological role in vertical stratif ication of microorganisms in microbial mats and water columns. It plays a c entral role in the behavior of magnetotactic bacteria and also appears to b e important in plant-microbe interactions.