Microbial Study Reveals Sophisticated Sensory Response

All known biological sensory systems, including the familiar examples of the five human senses – vision, hearing, smell, taste and touch – have one thing in common: when exposed to a sustained change in sensory input, the sense eventually acclimates and notices subsequent changes without continuing to compare each new change with the initial condition. This autonomous tuning of perceptions, known as sensory adaptation, has been recognized by scientists for more than a century, but a new study has demonstrated that even a simple microbe can achieve this feat with surprising sophistication. In a paper appearing the week of August 1 in the Proceedings of the National Academy of Sciences online, researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) and the Massachusetts Institute of Technology (MIT) describe, for the first time, a biological system in which sensory adaptation is so precise that behavior remains identical even in ever-changing "background" conditions. The researchers' system of choice is the bacterium Escherichia coli; they studied how this microbe's sensing of food alters its swimming behavior, or chemotaxis.

The new research is a collaboration between a Dutch team led by AMOLF group leader Tom Shimizu and an MIT team led by Roman Stocker, a professor in the Department of Civil and Environmental Engineering. Other team members are Milena D. Lazova, a graduate student in biophysics at AMOLF who is lead author of the paper; Tanvir Ahmed, who completed his Ph.D. studies at MIT in June; and Domenico Bellomo, an electrical engineer and systems biologist at the Delft University of Technology.

"This bacterial system offers a unique opportunity in the study of biological sensory processes," Shimizu said. "Its simplicity allows us to connect the molecular mechanisms, responsible for signal reception and processing, directly to how the organism behaves."

Much as animals often depend on the sense of smell to find food, the microscopic bacteria — each a single cell measuring only 2-4 micrometers in length — rely on a chemical sensing system to locate nutrients. To characterize this sensory response, Shimizu's group took advantage of a physical phenomenon known as Förster resonance energy transfer (FRET) that allows the monitoring of molecular interactions inside living cells using optical measurements.

Stocker's group conducted experiments using microfluidic devices – fluid channels of microscopic dimensions that allow precise control over the physical and chemical environment — to characterize changes in the bacteria's chemotaxis. Both types of experiments showed that when the size of the gradients in nutrient abundance was increased or decreased by the same factor as the changes in the background level of nutrients, the bacteria responded identically.