How Bacteria Swim in the World Without Inertia?

Dr. Linnea S. Komarov¹, Dr. Arjun Bhattacharya², Prof. Celeste Yamamoto³

¹ Department of Microbial Biophysics, Eastwood Institute of Technology, New Carthage² Center for Theoretical Biomechanics, Orion National University, Republic of Altamira³ School of Nanoscience and Quantum Engineering, Takeda University, Neo Tokyo

Abstract

Bacteria operate in a physical realm where viscous forces overwhelmingly dominate inertial effects, characterized by extremely low Reynolds numbers. In such environments, traditional propulsion strategies fail, necessitating unique adaptations for motility. This paper explores the hydrodynamic principles governing bacterial swimming, emphasizing the constraints imposed by low Reynolds numbers and the innovative mechanisms bacteria employ to navigate their viscous world. We discuss the implications of Purcell's scallop theorem, the role of flagellar rotation, and the adaptations observed in various bacterial species to overcome these physical challenges [10.1038/ncomms6119].

Introduction

At microscopic scales, the physics of locomotion differs fundamentally from our macroscopic experiences. Bacteria, typically a few micrometers in size, operate in a realm where the Reynolds number—a dimensionless quantity expressing the ratio of inertial to viscous forces—is exceedingly low, often around 10⁻⁴. In this regime, viscous forces overshadow inertia, rendering conventional propulsion methods ineffective. Understanding how bacteria achieve motility under these constraints provides insights into both fundamental physics and potential applications in nanotechnology and medicine [10.1038/s41578-021-00302-5].

Hydrodynamics at Low Reynolds Numbers

In low Reynolds number environments, the Navier-Stokes equations simplify to the Stokes equations, where inertial terms are negligible. This leads to time-reversible flow fields, meaning that any reciprocal motion—movements that are identical in forward and reverse—results in no net displacement. This principle is encapsulated in Purcell's scallop theorem, which states that a swimmer employing reciprocal motion cannot achieve propulsion in a Newtonian fluid at low Reynolds numbers. Consequently, bacteria must utilize non-reciprocal movements to swim effectively [10.1038/ncomms6119].

Bacterial Propulsion Mechanisms

To navigate their viscous environment, bacteria have evolved specialized structures and movements. Many species, such as Escherichia coli, use helical flagella that rotate to generate thrust. The rotation of these flagella breaks time-reversal symmetry, allowing for propulsion despite the constraints of the scallop theorem. Other bacteria employ different strategies; for instance, some use type IV pili to pull themselves forward in a process known as "twitching motility." These adaptations highlight the diverse solutions bacteria have developed to overcome the challenges of low Reynolds number locomotion [10.1038/s41578-021-00302-5].

Implications and Applications

Understanding bacterial motility in low Reynolds number environments has broader implications beyond microbiology. It informs the design of artificial microswimmers and nanorobots intended for medical applications, such as targeted drug delivery. By mimicking bacterial propulsion mechanisms, engineers can develop devices capable of navigating the human body's viscous fluids. Additionally, studying these systems enhances our comprehension of fluid dynamics at small scales, contributing to advancements in various scientific fields [10.1038/s41578-021-00302-5].

Conclusion

Bacteria have adapted to their low Reynolds number world by evolving non-reciprocal propulsion mechanisms that circumvent the limitations imposed by viscous dominance and the scallop theorem. These adaptations not only ensure their survival and proliferation but also inspire technological innovations in microscale engineering. Continued research into bacterial motility will further elucidate the principles of life at small scales and drive the development of novel applications in science and medicine.

References

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