Bacterial reconcentration in microchannels of varying cross-section and flow

Upstream swimming bacteria in unidirectional flows have been associated with spreading infections in tubular organ systems such as human respiratory and urinary tracts. Persistent swimmers such as Escherichia coli can rapidly contaminate capillaries by swimming against the flow, irrespective of its speed. Here, we report on anomalous accumulation of swimming bacteria depending on the cross-section of the microfluidic conduits: narrower microchannels subjected to flow result in increased bacterial concentrations with respect to wider channels that are directly connected. Our experimental findings can be rationalized by means of stochastic simulations accounting for bacterial speed and interactions with flow and confining structures.  This newly observed phenomenon can one day be utilized to predict or avoid the onset of bacterial settling in biological or soil networks.