Hydrodynamic flow and concentration gradients in the gut enhance neutral bacterial diversity

TL;DR

This study models how hydrodynamic flow and concentration gradients in the gut increase the fixation probability of neutral bacterial mutants, highlighting the importance of spatial structure in microbial evolution.

Contribution

It introduces a minimal spatial model of the gut that accounts for flow and gradients, revealing increased fixation probabilities compared to well-mixed systems.

Findings

Fixation probability is higher with spatial gradients.

Fixation probability becomes independent of total population size.

Active population size determines effective fixation probability.

Abstract

The gut microbiota features important genetic diversity, and the specific spatial features of the gut may shape evolution within this environment. We investigate the fixation probability of neutral bacterial mutants within a minimal model of the gut that includes hydrodynamic flow and resulting gradients of food and bacterial concentrations. We find that this fixation probability is substantially increased compared to an equivalent well-mixed system, in the regime where the profiles of food and bacterial concentration are strongly spatially-dependent. Fixation probability then becomes independent of total population size. We show that our results can be rationalized by introducing an active population, which consists of those bacteria that are actively consuming food and dividing. The active population size yields an effective population size for neutral mutant fixation probability in the gut.