  Colloids coupled to a bath of swimming cells generically display enhanced
diffusion. This transport dynamics stems from a subtle interplay between the
active and passive particles that still resists our understanding despite
decades of intense research. Here, we tackle the root of the problem by
providing a quantitative characterisation of the single scattering events
between a colloid and a bacterium. Based on our experiments, we build a minimal
model that quantitatively predicts the geometry of the scattering trajectories,
and enhanced colloidal diffusion at long times. This quantitative confrontation
between theory and experiments elucidates the microscopic origin of enhanced
transport. Collisions are solely ruled by stochastic contact interactions
responsible both for genuine anomalous diffusion at short times and enhanced
diffusion at long times with no ballistic regime at any scale.
