Symbiotic Bacteria Use Flagella to Drill Through Tight Biological Passages

How can bacteria squeeze through spaces narrower than a human hair is thick? A research team in Japan led by Dr. Daisuke Nakane and Dr. Tetsuo Kan at the University of Electro-Communications, Dr. Hirofumi Wada at Ritsumeikan University, and Dr. Yoshitomo Kikuchi at National Institute of Advanced Industrial Science and Technology have revealed the answer: they drill their way through.

The study, published in Nature Communications, shows that certain symbiotic bacteria wrap their rotating flagella-the helical tails used for swimming-around their cell bodies to form a "screw thread." This configuration lets them propel forward through one-micrometer-wide passages, such as those inside insect guts, that would otherwise trap or immobilize them.

To visualize this remarkable motion, the researchers built a quasi-one-dimensional microfluidic device that reproduces the geometry of an insect's gut "sorting organ." Under the microscope, bacteria called Caballeronia insecticola were seen advancing smoothly through the narrow channels by repeatedly wrapping and unwrapping their flagella. In contrast, related species that could not perform this wrapping remained stuck.

Computer simulations confirmed the physical advantage of the wrapping mode: in confined spaces, normal flagellar rotation simply stirs the fluid, while wrapping flagella efficiently generates its propulsion in the walls like a rotating corkscrew, efficiently pushing the cell forward.

The team further demonstrated that a flexible joint known as the hook, which connects the flagellar motor to its filament, is key to enabling this motion. When researchers swapped the hook genes from a "wrapping" species to a "non-wrapping" species, the ability to move through confined spaces-and even to infect their host insects-was lost accordingly.

This discovery highlights a new physical survival strategy among bacteria, revealing how simple microorganisms can exploit mechanical principles to navigate complex environments. Beyond its biological significance, understanding these micro-scale drilling motions may inspire microrobots capable of moving through viscous or crowded environments such as tissues or filtration systems.

Flagellar wrapping shows how life solves mechanical problems in elegant, unexpected ways. It's a microscopic version of engineering ingenuity, evolved by nature itself."

Dr. Daisuke Nakane, University of Electro-Communications