# Limits to the evolution of metabolic dependency in spatially structured microbial communities

## TL;DR

This study explores how auxotrophic bacteria survive in structured communities by analyzing amino acid sharing and growth dynamics.

## Contribution

The study combines single-cell imaging and modeling to reveal how auxotrophs invade and persist in spatially structured microbial communities.

## Key findings

- Auxotrophs grow slowly due to limited amino acid leakage from wildtype neighbors.
- Clustering of auxotrophs intensifies local amino acid depletion and hinders their growth.
- A growth advantage occurs only when amino acid leakage exceeds a threshold or when amino acids are externally supplied.

## Abstract

In microbial communities, evolutionary processes can lead to loss of biosynthetic pathways, creating metabolic dependencies. The Black Queen Hypothesis suggests that such gene loss can confer a fitness advantage by reducing metabolic burden. However, how these dependencies evolve at the level of individual cells in spatially structured communities remains poorly understood. We used a combination of microfluidic single-cell imaging and mathematical modeling to examine the early fate of auxotrophic mutants within E. coli populations. We found that without external amino acids, auxotroph growth is strongly constrained by low amino acid leakage from wildtype neighbors, and further reduced when they form local clusters that drain this limited amino acid pool. A growth advantage was only observed when amino acids were added or when leakage from wildtypes exceeded a threshold. Together, our results reveal insights into determinants of mutant invasion fitness and the trade-offs between reducing metabolic costs and maintaining metabolic autonomy.

•Single-cell imaging reveals how auxotrophs fare in spatially structured populations•Auxotrophs grow slowly due to low amino acid leakage from wildtype neighbors•Mutant clustering intensifies local amino acid depletion and reduces auxotroph growth•Model identifies the leakage threshold required for auxotroph invasion from rare

Single-cell imaging reveals how auxotrophs fare in spatially structured populations

Auxotrophs grow slowly due to low amino acid leakage from wildtype neighbors

Mutant clustering intensifies local amino acid depletion and reduces auxotroph growth

Model identifies the leakage threshold required for auxotroph invasion from rare

Cell biology; Evolutionary biology; Microbiology; Systems biology