# Geometry and mechanics of microdomains in growing bacterial colonies

## TL;DR

This study combines simulations and experiments to reveal how bacterial colonies self-organize into microdomains driven by mechanical forces, with implications for understanding colony growth and structure.

## Contribution

It introduces a comprehensive biomechanical model explaining microdomain formation in bacterial colonies, validated by experimental data.

## Key findings

- Microdomains emerge due to a balance of steric and active stresses.
- The domain size scales with the square root of the ratio of stiffness to stress.
- Experimental data on E. coli colonies agree with theoretical predictions.

## Abstract

Bacterial colonies are abundant on living and nonliving surfaces and are known to mediate a broad range of processes in ecology, medicine, and industry. Although extensively researched, from single cells to demographic scales, a comprehensive biomechanical picture, highlighting the cell-to-colony dynamics, is still lacking. Here, using molecular dynamics simulations and continuous modeling, we investigate the geometrical and mechanical properties of a bacterial colony growing on a substrate with a free boundary and demonstrate that such an expanding colony self-organizes into a "mosaic" of microdomains consisting of highly aligned cells. The emergence of microdomains is mediated by two competing forces: the steric forces between neighboring cells, which favor cell alignment, and the extensile stresses due to cell growth that tend to reduce the local orientational order and thereby distort the system. This interplay results in an exponential distribution of the domain areas and sets a characteristic length scale proportional to the square root of the ratio between the system orientational stiffness and the magnitude of the extensile active stress. Our theoretical predictions are finally compared with experiments with freely growing E. coli microcolonies, finding quantitative agreement.