Our senses act in partnership with our complex brains to enable us to identify food sources and go after them. But how do simple, single-celled Escherichia coli (E. coli) bacteria identify the presence and position of food sources? They do it with chemistry. Clusters of proteins in the bacteria’s outer membrane can sense very small concentrations of nutrients in the cell’s watery surroundings. Once nutrient compounds are detected, the receptor molecules release other proteins into the interior of the bacterial cell that undergo simple reactions and eventually find their way to the molecular motor that drives the cell’s spinning tail or flagellum. Whereas the normal rotation of the tail moves the bacterium along a straight path, the effect of these chemical signals changes the rotation of the flagellum causing the bacterium to tumble in the direction of the nutrient source. The bacterium follows the concentration gradient – the concentration of nutrient molecules that increases with proximity to its source.
Membrane-bound receptor proteins detect the presence of nutrients (A), triggering the release of a signal molecule (B), that reacts with proteins associated with the bacterium’s flagellum (C), causing a change in its direction of rotation (D), which propels the organism towards increasing concentrations of nutrients. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
Learn more about how E. coli bacteria translate sensory signals into movement in the iBiology video lecture entitled, “Marvels of Bacterial Behavior – Molecular Machinery.”Edit Summary
“Efficient biological signal processing often requires complex spatial organization of the signaling machinery…the bacterial chemotaxis system, which directs the movement of cells towards or away from sugars, amino acids, and many other soluble molecules. In Escherichia coli, five types of transmembrane chemoreceptors form trimers of dimers, which cluster into large complexes containing tens of thousands of proteins. Receptor clustering enables cooperative interactions between receptors, contributing to a bacterium’s ability to sense nanomolar concentrations of chemicals and small fractional changes in chemical concentrations over a wide range…CheA [one of the proteins central to bacterial chemotaxis] transduces signals from membrane receptors to the cytoplasmic response regulator CheY, which diffuses to flagellar motors and modulates their direction of rotation.” (Greenfield 2009: 1)
“CheY is the chemotactic response regulator, which transduces signals from the receptors to flagellar motors.” (Greenfield 2009: 2)
“[R]ecent in vitro data suggest that different densities of receptors have different kinase and methylation rates, suggesting that the chemotaxis network may adjust its kinase activity based on the local concentration of receptors.” (Greenfield 2009: 8)