Bacterial Chemotaxis with a Moving Target

Most chemotaxis studies so far have been conducted in a quiescent fluid with a well-defined chemical gradient. Such experiments may be appropriate for studying enteric bacteria, such as \textit{Escherichia coli}, but the environment it provides is very different from that typically encountered by marine bacteria. Herein we describe an experiment in which marine bacterium \textit{Vibrio alginolyticus}is subject to stimulation by a small moving target. A micropipette of the tip size \textless 1 ?m is used to slowly release a chemoattractant, serine, at different concentrations. The pipette is made to move with different patterns and speeds, ranging from 0 to 100 ?m/s; the latter is about twice the bacterial swimming speed. We found that if the pipette is moved slowly, with 1/4 of bacterial swimming speed, cells accumulate near the tip region but when it is moved with speed greater than 1/2 the bacterial swimming speed, cells trail behind the pipette over a large distance. The behaviors observed in \textit{V. alginolyticus}are significantly different from \textit{E. coli}, suggesting that the former is a better chemotaxer in a changing environment.