Using ratchets and sorters to fractionate motile cells of Escherichia coli by length

This paper describes the fabrication of a composite agar/PDMS device for enriching short cells in a population of motile Escherichia coli. The device incorporated ratcheting microchannels, which directed the motion of swimming cells of E. coli through the device, and three sorting junctions, which isolated successively shorter populations of bacteria. The ratcheting microchannels guided cells through the device with an average rate of displacement of (32 ± 9) μm s⁻¹. Within the device, the average length of the cells decreased from 3.8 μm (Coefficient of Variation, CV: 21%) at the entrance, to 3.4 μm (CV: 16%) after the first sorting junction, to 3.2 μm (CV: 19%) after the second sorting junction, to 3.0 μm (CV: 19%) after the third sorting junction.