Escherichia coli swim on the right-hand side

The motion of peritrichously flagellated bacteria close to surfaces is relevant to understanding the early stages of biofilm formation and of pathogenic infection(1-4). This motion differs from the random-walk trajectories(5) of cells in free solution. Individual Escherichia coli cells swim in clockwise, circular trajectories near planar glass surfaces(6,7). On a semi-solid agar substrate, cells differentiate into an elongated, hyperflagellated phenotype and migrate cooperatively over the surface(8), a phenomenon called swarming. We have developed a technique for observing isolated E. coli swarmer cells(9) moving on an agar substrate and confined in shallow, oxidized poly( dimethylsiloxane) ( PDMS) microchannels. Here we show that cells in these microchannels preferentially 'drive on the right', swimming preferentially along the right wall of the microchannel ( viewed from behind the moving cell, with the agar on the bottom). We propose that when cells are confined between two interfaces - one an agar gel and the second PDMS they swim closer to the agar surface than to the PDMS surface ( and for much longer periods of time), leading to the preferential movement on the right of the microchannel. Thus, the choice of materials guides the motion of cells in microchannels.