The Secret Superpower of Bacterial Swimming

Imagine trying to swim through a pool filled with honey while someone constantly jiggles it—this is the everyday reality for bacteria navigating their microscopic worlds. ² Flagellated bacteria like E. coli inhabit environments far more complex than the simple liquids in our laboratories—they swim through biological environments teeming with proteins, mucus, and other particles that create fluids with strange, non-Newtonian properties.

For decades, scientists have been puzzled by a fascinating phenomenon: bacteria somehow swim faster in these complex fluids than in simple water-like mediums. This mystery isn't just academic curiosity—understanding how microbes navigate complex environments has profound implications for medical treatments, environmental science, and the development of microscopic robots that could one day deliver drugs inside our bodies.

Recent groundbreaking research has finally uncovered the secret behind this bacterial superpower, and the answer challenges everything we thought we knew about microscopic locomotion. By studying bacteria in carefully engineered colloidal suspensions, scientists have discovered a universal mechanism that explains not only why bacteria swim faster in complex fluids but also why their paths become straighter—revealing nature's ingenious solution to moving through chaotic environments. ²

In 2019, a team of researchers from the University of Minnesota decided to approach the problem from a different angle. Instead of using polymer solutions with their complicated molecular dynamics, they turned to well-controlled colloidal suspensions—tiny solid particles suspended in liquid. This experimental approach offered two significant advantages:

1. Precise size control—colloidal particles could be manufactured in specific sizes from 50 nanometers to 1 micrometer
2. Simpler physics—unlike polymers that stretch and deform, colloidal particles behave as hard spheres with predictable interactions

The research team, led by Shashank Kamdar, Lorraine F. Francis, and Xiang Cheng, presented their initial findings at the 2019 APS Division of Fluid Dynamics Meeting, revealing surprising similarities between bacterial behavior in colloidal suspensions and polymer solutions. ^{1 5}

Their subsequent paper, published in Nature in 2022, delivered a groundbreaking conclusion: the colloidal nature of complex fluids—not unique polymer dynamics—was primarily responsible for the enhanced bacterial motility. This discovery challenged six decades of established thinking and provided a unified explanation for bacterial swimming in diverse complex fluids.

The University of Minnesota team realized that the key to understanding speed enhancement lay in explaining the reduced wobbling. They developed a simple yet powerful physical model based on hydrodynamic interactions between bacteria and colloidal particles:

When a bacterium approaches a colloidal particle, the hydrodynamic interaction generates a subtle torque on the bacterial body. This torque acts to align the flagellar bundle more precisely with the cell body, reducing the misalignment angle that causes wobbling in Newtonian fluids.

With less energy wasted on erratic side-to-side motion, more propulsion goes toward forward movement, resulting in greater efficiency and higher speed. The model provides a parameter-free expression that quantitatively predicts bacterial speed enhancement in both colloidal and polymeric fluids. ^{2 8}

This mechanism successfully explains why both colloids and polymers produce similar effects—both represent discrete obstacles that interact hydrodynamically with swimming bacteria, regardless of their specific material properties.