Collective Dynamics of Model Pili-Based Twitcher-Mode Bacilliforms

Abstract

Pseudomonas aeruginosa, like many bacilliforms, are not limited only to swimming motility but rather possess many motility strategies. in particular, twitching-mode motility employs hair-like pili to transverse moist surfaces with a jittery irregular crawl. twitching motility plays a critical role in redistributing cells on surfaces prior to and during colony formation. We combine molecular dynamics and rule-based simulations to study twitching-mode motility of model bacilliforms and show that there is a critical surface coverage fraction at which collective effects arise. Our simulations demonstrate dynamic clustering of twitcher-type bacteria with polydomains of local alignment that exhibit spontaneous correlated motions, similar to rafts in many bacterial communities. Active matter possesses the potential to bridge between physics and biology. Like living systems, manufactured active systems maintain far-from-equilibrium states by autonomously drawing energy from the surroundings to fuel non-thermal processes. Furthermore, active systems exhibit many of the characteristic traits of biological materials, such as spontaneous motion, self-organization and complex spatio-temporal dynamics. Communities of model bacteria, such as Pseudomonas aeruginosa, are excellent biological examples of out-of-equilibrium systems. These relatively simple living systems serve as a biophysical study of active matter in which collectivity arising from bio-mechanical action can perform essential biological roles. Theories and simulations have approached such bacterial systems by simplifying or omitting all but the most essential, lowest-order physical traits of these microbes, as well as biological complexities. From the very first considerations of active matter, self-propulsion and local alignment were identified as the fundamental components necessary for collective dynamics to emerge from active particles 1,2. Simulations of self-propelled rods and their continuum limit of active nematics have been particularly important to the field 3-14 , as recently reviewed in ref. 15. However, the universality of behaviors exhibited by active systems is still a matter of debate 16,17 and it cannot simply be taken for granted that the collective dynamics of Vicsek boids 1 , active Brownian particles 18-20 or self-propelled rods are directly inherited by microbial motility strategies. Indeed it is known that what might appear to be higher-order details can qualitatively alter the large-scale dynamics. For example, while self-propelled rods and other active colloids commonly exhibit pronounced clustering 21 , which can be explained by motility-induced phase separation or other theoretical approaches 22-24 , swimming microbes can behave as homogeneous fluids on the scales of mesoscale active turbulence 25 , with simulations suggesting that the details of hydrodynamic interactions are essential for differentiating these large-scale swimmer properties 20,26-28. Various modes of swimming motility, including but not limited to pushing, pulling, squi...

Microtubules Enhance Mesoscale Effective Diffusivity in the Crowded Metaphase Cytoplasm

“…A value of a 2 >0 indicates that there is an increased frequency of large displacements relative to a normal distribution. This regime (a 2 >0) has been reported for dynamics ranging from active transport to changes in confinement (Ahmed and Saif, 2014;Nagel et al, 2020;Parry et al, 2014;Wang et al, 2009).…”

Mesoscopic Simulations of Active-Nematics

“…Different studies have attempted to model the microscopic details to a greater or lesser extent. For example, simulations of simple self-propelled rods in the dry limit [16] have found many of the same properties as more detailed simulations of substrate-crawling bacilliforms [19]. Similarly, self-propelled rods in the wet limit [20,21] agree with simulations of swimming bacteria [22,23].…”

“…where d = 2 is the dimension and ∆r k is the k'th moment of the distribution [19]. When a distribution is purely Gaussian χ NGM = 0.…”

Mesoscopic simulations of active nematics

“…Different studies have attempted to model the microscopic details to a greater or lesser extent. For example, simulations of simple self-propelled rods in the dry limit ( 12 ) have found many of the same properties as more detailed simulations of substrate-crawling bacilliforms ( 14 ). Similarly, self-propelled rods in the wet limit ( 15 ) agree with simulations of swimming bacteria ( 16 ).…”