New research from Arizona State University shows that bacteria can travel in unexpected ways even when their usual propulsion system fails. Normally, bacteria move using flagella, slender, whip-like structures that spin to push the cells forward. The new studies reveal that microbes can still spread across surfaces without these structures.
Movement is critical for bacteria. It allows them to gather into communities, explore new environments, and escape harmful conditions. Learning how bacteria move may help scientists design better strategies to prevent infections.
In the first study, researcher Navish Wadhwa and his team found that salmonella and E. coli can migrate across moist surfaces even when their flagella are disabled. The bacteria generate motion through their metabolism. When they ferment sugars, they create tiny outward flowing currents across the wet surface. These flows slowly push the bacterial colony outward, similar to leaves drifting along a thin stream.
Researchers named this newly identified movement "swashing." The discovery could help explain how disease causing microbes manage to colonize medical devices, wounds, and food processing equipment. By understanding how bacterial metabolism drives this type of motion, scientists may be able to slow or stop it by altering environmental conditions such as pH or sugar levels.
"We were amazed by the ability of these bacteria to migrate across surfaces without functional flagella. In fact, our collaborators originally designed this experiment as a 'negative control,' meaning that we expected (once rendered) flagella-less, the cells to not move," Wadhwa says. "But the bacteria migrated with abandon, as if nothing were amiss, setting us off on a multiyear quest to understand how they were doing it.
"It just goes to show that even when we think we've got something figured out, there are often surprises waiting just under the surface, or in this case, above it."
Wadhwa is a researcher with the Biodesign Center for Mechanisms of Evolution and assistant professor with the Department of Physics at ASU. The study appears in the Journal of Bacteriology and was selected as an Editor's Pick, highlighting its significance.
Sugar Fueled Swashing
The swashing effect begins when bacteria consume fermentable sugars such as glucose, maltose, or xylose. During fermentation, the microbes release acidic by products including acetate and formate. These compounds pull water toward the colony from the surrounding surface, creating tiny currents that push the cells outward.
Fermentable sugars are required for this movement. Without them, bacteria cannot produce the fluid flows needed for swashing. Sugar rich environments inside the body, such as mucus, could therefore make it easier for harmful bacteria to spread and trigger infections.
Scientists also tested what happens when surfactants, detergent-like molecules, are added to the colonies. These compounds stopped swashing completely. However, the same chemicals did not interfere with swarming, another type of bacterial movement powered by flagella that enables microbes to rapidly spread across wet surfaces. This difference suggests the two behaviors rely on separate physical mechanisms. It also hints that surfactants might someday be used to control bacterial movement depending on whether microbes are swashing or swarming.
The discovery that bacteria can colonize surfaces even when their normal swimming machinery fails has important health implications. Some microbes could spread across medical catheters, implants, or hospital equipment through swashing. Simply blocking flagella might not prevent that spread. Instead, treatments may need to target the metabolic processes that drive the fluid currents.
E. coli and salmonella are both well known causes of foodborne illness. Recognizing that these bacteria can spread through passive fluid flows may help improve sanitation strategies in food processing facilities. Because swashing depends on fermentation and acidic by products, altering factors such as surface pH or sugar levels could limit bacterial growth. The study found that even modest changes in acidity could influence how bacteria move.
Similar conditions may also exist inside the human body. Moist environments such as gut mucus, wound fluids, or the urinary tract provide surfaces where bacteria could spread through swashing, even when their flagella are not functioning effectively.
A Molecular Gear System for Bacterial Movement
A second study examined a different group of microbes called flavobacteria. Unlike E. coli, these bacteria do not swim. Instead, they travel along environmental and host surfaces using a specialized machine known as the type 9 secretion system, or T9SS. This system powers a molecular conveyor belt that moves along the surface of the cell.
Under normal conditions, the T9SS allows flavobacteria to glide across surfaces. The mechanism works by moving an adhesive coated belt around the outside of the cell, pulling the bacterium forward in a motion that resembles a microscopic snowmobile.
The researchers discovered that a protein within this system, called GldJ, acts as a type of gear shifter that controls the direction of the motor. When a small portion of GldJ is removed, the motor reverses its rotation from counterclockwise to clockwise. This change alters the bacterium's direction of travel.
The study describes this molecular gear mechanism in detail and shows how it allows bacteria to adjust their movement in response to complex environments. This ability may provide an evolutionary advantage by helping microbes navigate surfaces more effectively.
Implications for Human Health and Microbiome Research
The T9SS system influences more than just bacterial motion. It can also affect human health in different ways depending on the microbial community involved.
In the oral microbiome, bacteria that contain the T9SS system have been linked to gum disease. The proteins they release can trigger inflammation in the mouth and may also contribute to conditions such as heart disease and Alzheimer's.
In contrast, T9SS activity in the gut microbiome can be beneficial. Proteins secreted through this system can protect antibodies from breaking down, which strengthens immune defenses and may improve the effectiveness of oral vaccines.
Understanding how this molecular gearbox works could help researchers develop ways to stop bacteria from forming biofilms, slimy communities that cause infections and contaminate medical devices. At the same time, scientists may be able to harness these mechanisms to support beneficial microbes and design targeted microbiome therapies.
"We are very excited to have discovered an extraordinary dual-role nanogear system that integrates a feedback mechanism, revealing a controllable biological snowmobile and showing how bacteria precisely tune motility and secretion in dynamic environments," Shrivastava says. "Building on this breakthrough, we now aim to determine high-resolution structures of this remarkable molecular conveyor to visualize, at atomic precision, how its moving parts interlock, transmit force and respond to mechanical feedback. Unraveling this intricate design will not only deepen our understanding of microbial evolution but also inspire the development of next-generation bioengineered nanomachines and therapeutic technologies."
Shrivastava is a researcher with the Biodesign Center for Fundamental and Applied Microbiomics, the Biodesign Center for Mechanisms of Evolution, and assistant professor with ASU's School of Life Sciences. The findings appear in the journal mBio.
Multiple Strategies Help Bacteria Spread
At first glance, the two discoveries -- fluid surfing and molecular gear-shifting, seem worlds apart. However, both highlight how bacteria have evolved a range of unexpected strategies to move and spread. The more movement options microbes possess, the harder they are to control.
These findings also suggest that new approaches may be needed to combat bacterial infections. Many traditional strategies focus on disabling flagella. But these studies show that bacteria can continue spreading even without them.
The research points to the importance of controlling the environments where bacteria live. Factors such as sugar availability, pH levels, and surface chemistry could play a major role in limiting bacterial movement. Interfering with molecular systems like the T9SS gearbox could also prevent bacteria from moving and from releasing harmful proteins that contribute to disease.
