Determination of Bacterial Chemotaxis Response Functions by Optical Trapping

A two-lobe response function is considered as a manifestation of temporal signal comparison in bacterial chemosensing. The second lobe in the response function appears as a result of adaptive behavior of the underlying signaling network, which allows bacteria to stay sensitive over a wide range of background signal levels. It has been argued that this two-lobe response reflects the dual requirements of the bacteria to taxis along a chemical gradient and to localize once the top of the gradient is reached. Calculations based on the run-tumble motility pattern of Escherichia coli showed that the second lobe improved the bacterium's localization capability. Intrigued by a recently observed run-reverse-flick motility cycle of a marine bacterium Vibrio alginolyticus, we investigate the motility-response relationship in this bacterium. Using a novel optical trapping technique, we measure the response of V. alginolyticus to an impulsive stimulus of chemoattractant serine. By exploiting an asymmetry in the rotation of the polar flagellum, we are able to determine for the first time how the bacterium responds to chemical stimuli while swimming forward or backward. Our measurements suggest that this marine bacterium regulates its forward and backward swimming intervals differently, exhibiting behaviors that is consistent with an exploration-exploitation strategy.\ud In our measurements, we also find that the cell-body Ω(t) and the flagellar ω(t) rotational angular frequencies oscillate in time and are in synchrony with the forward and backward swimming intervals. Unexpectedly, Ω(t) and ω(t) are found to be anticorrelated in that the cell body rotates slower in the forward direction than in the backward direction, Ωf