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.
Source: ScienceDaily
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