Flagella – the tiny whippy “hairs” on bacteria – are mechanical wonders, the only known biological structures that act like reversible rotary motors. Bacterial pathogens use these flagellar motors like a propeller to swim in hosts and promote infection. Some bacteria are covered with long flagella while others have only one or two short flagella on their ends. Until recently, it was unknown how some bacteria produced long filaments and others made short ones.

Now, a team of UT Southwestern researchers have found that a protein called FlaG controls the length of the flagellum in Campylobacter jejuni, the leading cause of diarrheal disease in the U.S.

This study, published in the Proceedings of the National Academy of Sciences, is the first to find a natural antagonist to flagellum construction. The research holds potential for creation of a molecule that could fight infections by interfering with flagellin secretion, and thus hamstringing a bacterium’s ability to swim.

The flagellar filament forms this ‘whip’-like structure on the bacterium’s surface and contains thousands of units of the protein flagellin. An “export gate” for a secretion system at the bottom of the filament sends flagellin units through the hollow filament, where they’re added to the tip.

Researchers found that FlaG binds to part of the export gate and stops secretion of flagellin. They also discovered that a similar protein limits flagellar length in Vibrio cholerae, which causes cholera, and Pseudomonas aeruginosa, which causes respiratory infections, sepsis, and other serious infections.

“It’s been at least 20 years that we’ve known that something’s happening with FlaG, but no one really dived deep to see how it functions,” said lead researcher Alexis Waller, a graduate student in UT Southwestern’s Molecular Microbiology Graduate Program.

“This was brand new for flagella because they weren’t known to have a gatekeeper,” said David Hendrixson, Ph.D., UTSW Professor of Microbiology and Ms. Waller’s mentor.

However, Dr Hendrixson said, the finding fits in with how some pathogens operate. Other bacterial pathogens use a structure called an injectisome to insert proteins into host cells via a short tube. Gatekeepers work on the injectisomes to control access of these injected proteins for a similar export gate in the injectisome.

C. jejuni with normally short flagellar propellers (inset) and a C. jejuni lacking FlaG, causing the bacterium to make abnormally long flagellar propellers. The researchers in this study found how FlaG controls bacteria to produce short flagella rather than long flagella.

Ms. Waller fed their data into a program called AlphaFold 3, which predicted that a stretch of six amino acids on FlaG would interact with a specific region of the FlhA protein that forms part of the export gate. When she created a series of C. jejuni strains with various FlhA mutations so that FlaG could no longer bind, Ms. Waller found that these bacteria built significantly longer flagella – consistent with the hypothesis that FlaG binding to FlhA inhibits the length. Further investigation showed that FlaG binding to FlhA antagonizes the ability of a chaperone protein called FliS to deliver flagellin to the export gate.

Future studies will examine if FlaG can shorten flagellar filament lengths in bacteria that naturally lack its gene, which will show how broadly its potential could be to antagonize flagellar formation in different pathogenic bacteria.

Also contributing to the study was Deborah Ribardo, Ph.D., a Senior Research Scientist in the Hendrixson Lab. The work was supported by Public Health National Institutes of Health grants R21AI159140 and R01AI065539.