Hydrodynamic flow in the gut hinders the fixation of beneficial mutants but accelerates their invasion

Gut bacteria experience a spatially structured environment with hydrodynamic flow. Using a branching-process approach, we show that beneficial mutants arising randomly upon division have a reduced fixation probability compared to the well-mixed case. The suppression factor is set by the spatial heterogeneity of mutant fate across the gut. In contrast, the invasion of mutant lineages that avoid early stochastic extinction is accelerated in the gut compared to a matched well-mixed case. Stronger advective flow decreases fixation probabilities while accelerating mutant invasion.