How bacterial cells and colonies move on solid substrates

TL;DR

This study investigates bacterial motility on solid surfaces, combining experimental observations with stochastic and computational models to understand how pili interactions and friction influence movement of single cells and colonies.

Contribution

It introduces a stochastic model explaining single-cell persistence and highlights the role of sliding friction in colony motility, supported by detailed computational simulations.

Findings

Single-cell motility follows a persistent random walk with persistence length exceeding pili length.

Colony motility decreases with increasing colony size without friction considerations.

Sliding friction is essential to accurately model and explain colony movement.

Abstract

Many bacteria rely on active cell appendages, such as type IV pili, to move over substrates and interact with neighboring cells. Here, we study the motion of individual cells and bacterial colonies, mediated by the collective interactions of multiple pili. It was shown experimentally that the substrate motility of Neisseria gonorrhoeae cells can be described as a persistent random walk with a persistence length that exceeds the mean pili length. Moreover, the persistence length increases for a higher number of pili per cell. With the help of a simple, tractable stochastic model, we test whether a tug-of-war without directional memory can explain the persistent motion of single Neisseria gonorrhoeae cells. While the persistent motion of single cells indeed emerges naturally in the model, a tug-of-war alone is not capable of explaining the motility of microcolonies, which becomes weaker with increasing colony size. We suggest sliding friction between the microcolonies and the substrate as the missing ingredient. While such friction almost does not affect the general mechanism of single cell motility, it has a strong effect on colony motility. We validate the theoretical predictions by using a three-dimensional computational model that includes explicit details of the pili dynamics, force generation and geometry of cells.