# Mechanical interactions in bacterial colonies and the surfing probability of beneficial mutations

## TL;DR

This study investigates how mechanical interactions among bacteria in colonies influence their growth, movement, and evolution, particularly affecting the likelihood of beneficial mutations 'surfing' to prominence at the colony front.

## Contribution

It introduces a computer model linking physical cell properties to evolutionary outcomes, highlighting the role of mechanical interactions in bacterial evolution.

## Key findings

- Cell shape, elasticity, and friction affect surfing probability.
- Front roughness influences mutation surfing success.
- Experimental validation confirms model predictions.

## Abstract

Bacterial conglomerates such as biofilms and microcolonies are ubiquitous in nature and play an important role in industry and medicine. In contrast to well-mixed, diluted cultures routinely used in microbial research, bacteria in a microcolony interact mechanically with one another and with the substrate to which they are attached. Despite their ubiquity, little is known about the role of such mechanical interactions on growth and biological evolution of microbial populations. Here we use a computer model of a microbial colony of rod-shaped cells to investigate how physical interactions between cells determine their motion in the colony, this affects biological evolution. We show that the probability that a faster-growing mutant "surfs" at the colony's frontier and creates a macroscopic sector depends on physical properties of cells (shape, elasticity, friction). Although all these factors contribute to the surfing probability in seemingly different ways, they all ultimately exhibit their effects by altering the roughness of the expanding frontier of the colony and the orientation of cells. Our predictions are confirmed by experiments in which we measure the surfing probability for colonies of different front roughness. Our results show that physical interactions between bacterial cells play an important role in biological evolution of new traits, and suggest that these interaction may be relevant to processes such as de novo evolution of antibiotic resistance.