Salmonella Doesn’t Swim the Way We Thought

Salmonella causes millions of foodborne illnesses worldwide each year. Its ability to infect the gut depends on how well it can move: the bacteria must push through the mucus layer that lines the intestinal wall before reaching and colonizing the cells beneath. Flagellar motility is key to that process, and researchers have now looked more closely at how S. Typhimurium — one of the serovars most commonly linked to human foodborne illness — actually swims, with some results that were not anticipated.

How Salmonella Gets Around

S. Typhimurium is a rod-shaped bacterium with four to six flagella distributed around its surface. When those flagella all spin in the same direction, they bundle together into a kind of biological propeller and drive the cell forward. If one or more switches its rotation, the bundle breaks apart and the bacterium tumbles, reorienting before the next run. This “run-and-tumble” pattern is how the bacteria navigate their environment, and flagellar motility is what allows them to push through the gut’s mucus barrier.

Most prior tracking experiments have observed S. Typhimurium moving near a surface, a setup that improves observation time but may not reflect how they behave when moving without that constraint. Now, a study published in ACS Nanoscience Au reports what happens when the bacteria swim freely in solution.

A Surprisingly Wide Range of Motion

Using fluorescence microscopy and a custom single-particle tracking (SPT) framework, the team followed more than 200,000 bacterial tracks. By separating each cell’s movement into motion parallel to its long axis and motion across it, they could measure two things simultaneously: how actively a bacterium was moving, and how well its body was pointing in the direction it was traveling.

The researchers expected S. Typhimurium to resemble Escherichia coli, in which actively swimming bacteria stay fairly close to their direction of travel (within about 25°). However, active S. Typhimurium bacteria showed a broad range of orientations relative to their direction of travel, from near-full alignment to virtually none at all. Some cells were tracked at angles exceeding 110°. This range is wider than reported in related bacterial species.

The framework also distinguished between strains expressing different flagellin proteins. S. Typhimurium can produce two flagellin variants, FljB and FliC. Wild-type bacteria and those expressing only FliC swam in similar ways overall, but differed in how many cells in the population were actively moving. Bacteria lacking both flagellin types showed no active motility.

What This Tells Us About Bacterial Infection

The SPT approach the team developed is designed to work at high throughput and to account for the elongated, asymmetric shape of rod-like bacteria — something prior studies largely overlooked by treating cells as symmetric particles. The authors argue this gives a more complete picture of bacterial motion in bulk solution, rather than at a surface.

The findings contribute to a growing understanding of how flagellar mechanics shape S. Typhimurium’s behavior at the individual-cell level, which the authors note is critical for understanding the early steps of host invasion and infection. The method may prove useful for studying other motile bacteria whose swimming behavior in unconfined conditions remains poorly characterized.

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