Bacterial chemotaxis and the question of gain

The devil was always in the details. But as we accumulate more and more quantitative data on living cells, those diabolical details become increasingly finicky and numerical. Small differences that we would have cheerfully disregarded a decade ago as being due to “experimental error” or “differences in technique” suddenly become crucial—the nub of debate if not controversy. Sometimes, even, they reveal cracks in the smooth surface of current dogma and hint at unexplored layers beneath. Take, for example, the question of the “gain” or “amplification” shown by swimming bacteria when they respond to chemical attractants. Arguments about the size of this gain and whether it is consistent with the currently accepted model of signal transduction have been going on for years. Now, an innovative technique applied to the problem measures chemotactic gain with unparalleled accuracy (1). And yes, there really is something strange going on. The story begins 30 years ago when Howard Berg and Douglas Brown at the University of Colorado used a novel three-dimensional tracking microscope they had developed to follow the swimming of individual Escherichia coli bacteria (2). Swimming was found to consist of smooth “runs” interrupted roughly every second by transient “tumbles” (or “twiddles”). Chemotaxis—the ability of the cells to move toward distant sources of food molecules—was based on the suppression of tumbles in cells that happened by chance to be moving up the gradient. The impressive sensitivity of this process was later measured by applying attractants such as aspartate directly from a pipette onto bacteria tethered onto a coverslip (3). In this tethered-cell method, periodic reversals of rotation of individual flagellar motors are observed and the swimming performance recorded as the rotational bias—the fraction of time spent in counterclockwise rotation. The measurements of Segall et al. (3) revealed that the amplification, or gain, of …