  The expansion of biological species in natural environments is usually
described as the combined effect of individual spatial dispersal and growth. In
the case of aquatic ecosystems flow transport can also be extremely relevant as
an extra, advection induced, dispersal factor. There is a lack of reproducible
experimental studies on biological fronts of living organisms in controlled
streaming habitats. It is thus not clear if, and to which extent, the current
theoretical and experimental knowledge on advective-reactive-diffusive fronts
for chemical reactions can also apply to the expansion of biological
populations. We designed and assembled a dedicated microfluidic device to
control and quantify the expansion of populations of $E.coli$ bacteria under
both co-flowing and counter-flowing conditions, measuring the front speed at
varying intensity of the imposed flow. At variance with respect to the case of
autocatalytic reactions, we measure that almost irrespective of the
counter-flow velocity, the front speed remains finite at a constant positive
value. A simple model incorporating growth, dispersion and drift on finite-size
hard beads allows to explain this finding as due to a finite volume effect of
the bacteria. This indicates that models based on the
Fisher-Kolmogorov-Petrovsky-Piscounov equation (FKPP) that ignore the finite
size of organisms may be inaccurate to describe the physics of spatial growth
dynamics of bacteria.
