  Interaction of swimming bacteria with flows controls their ability to explore
complex environments, crucial to many societal and environmental challenges and
relevant for microfluidic applications as cell sorting. Combining experimental,
numerical and theoretical analysis, we present a comprehensive study of the
transport of motile bacteria in shear flows. Experimentally, we obtain with
high accuracy and for a large range of flow rates, the spatially resolved
velocity and orientation distributions. They are in excellent agreement with
the simulations of a kinematic model accounting for stochastic and
microhydrodynamic properties and in particular the flagella chirality.
Theoretical analysis reveals the scaling laws behind the average rheotactic
velocity at moderate shear rates using a chirality parameter and explains the
reorientation dynamics leading to a saturation at large shear rates from the
marginal stability of a fixed point. Our findings constitute a full
understanding of the physical mechanisms and relevant parameters of bacteria
bulk rheotaxis.
