Twitching Motility of Bacteria with Type IV Pili: Fractal Walks, First passage time and their Consequences on Microcolonies

TL;DR

This study models and analyzes the fractal and stochastic properties of bacterial twitching motility driven by Type IV pili, revealing how these dynamics influence microcolony formation and biofilm growth.

Contribution

It introduces coarse-grained models based on real trajectories to describe bacterial twitching, highlighting the fractal nature and crossover behavior of their movement.

Findings

NG exhibits ballistic and diffusive motion at different time scales

A characteristic persistent length optimizes biofilm growth

Bacterial colony morphologies display fractal characteristics

Abstract

A human pathogen, \textit{Neisseria gonorrhoeae} (NG), moves on surfaces by attaching and retracting polymeric structures called Type IV pili. The \textit{tug-of-war} between the pili results in a two-dimensional stochastic motion called \textit{twitching motility}. In this paper, with the help of real time NG trajectories, we develop coarse-grained models for their description. The \textit{fractal properties} of these trajectories are determined and their influence on \textit{first passage time} and formation of bacterial microcolonies is studied. Our main observations are as follows: (i) NG performs a fast ballistic walk on small time scales and a slow diffusive walk over long time scales with a long crossover region; (ii) There exists a characteristic persistent length $l_p^*$ which yields the fastest growth of bacterial aggregates or biofilms. Our simulations reveal that $l_{p}^{*} \sim L^{0.6}$, where $L\times L$ is the surface on which the bacteria move; (iii) The morphologies have distinct fractal characteristics as a consequence of the ballistic and diffusive motion of the constituting bacteria.