Spontaneous spatial sorting by cell shape in growing colonies of rod-like bacteria

TL;DR

This study uses computational modeling to show how cell shape influences spatial organization in bacterial colonies, revealing that elongated cells tend to segregate to the periphery, which may have evolutionary implications.

Contribution

It demonstrates how heritable cell shape variations affect spatial genetic structure and segregation in bacterial colonies through a computational 2D Brownian dynamics model.

Findings

Longer aspect ratio cells segregate to colony periphery

Bidisperse colonies show reduced intermixing and clustering

Nematic order correlates with cell segregation patterns

Abstract

Mechanical interactions among cells in a growing microbial colony can significantly influence the colony's spatial genetic structure and, thus, evolutionary outcomes such as the fates of rare mutations. Here, we computationally investigate how this spatial genetic structure changes as a result of heritable phenotypic variations in cell shape. By modeling rod-like bacterial cells as lengthening and dividing circo-rectangles in a 2D Brownian dynamics framework, we simulate the growth of a colony containing two populations with different aspect ratios. Compared to monodisperse colonies, such bidisperse colonies exhibit diminished intermixing between sub-populations when the less elongated cells are too short to nematically order, instead forming large clusters. We find that the cells with longer aspect ratio gradually segregate to the colony periphery. We present evidence that this demixing is related to nematic order in the bulk and to active nematic mixing dynamics near the periphery. These findings are qualitatively robust across different growth rate protocols and initial conditions. Because the periphery is often an advantageous position when nutrients are limited, our results suggest a possible evolutionary selective pressure of mechanical origin that favors large cell aspect ratio.