There are billions of bacteria around us and in our bodies, most of which are harmless or even helpful. But some bacteria such as E. coli and salmonella can cause infections. The ability to swim can help bacteria to seek out nutrients or to colonize parts of the body and cause infection.

Researchers have now provided fundamental insight into how this bacterial movement is powered, solving a yearlong mystery within the field.

'A lot of bacteria can move, or swim, because they have long threads, also known as flagella, which they can use to propel themselves forward. They do this by rotating these threads. The rotation is powered by a rotary motor, which again is powered by a protein complex known as the stator unit. This is all well known within our field. What we now show is how this stator unit powers the motor, which has been a mystery so far', says the group leader.

Quite surprisingly, the team shows that the stator unit itself is in fact also a tiny rotary motor. This tiny motor powers the large motor, which makes the threads rotate, causing the bacteria to move. The results contradict existing theories on the mechanism of the stator unit, and this new knowledge might be useful in the fight against bacteria-based diseases.

'Most researchers, including ourselves, actually thought that the technical mechanism and the architecture of the stator unit was quite different to what our study shows. Knowing the actual composition and function of this unit paves the way for therapeutic purposes. When we know what makes bacteria move, we might also be able to inhibit this movement and thereby stop it from spreading', says the author.

The researchers determined the structure of the stator unit complex by using cryo-electron microscopy. Working with this technique, they were able to elucidate its architecture, see how it is activated and provide a detailed model for how it powers rotation of the flagellar motor.

"The motor consists of two proteins: MotA and MotB. The MotB protein is anchored to the cell wall, and is surrounded by MotA proteins, which, upon dispersion of the ion motive force, rotates around MotB. The rotation of MotA in turn powers rotation of the large bacteria motor," says the author.

"Furthermore, our model shows how the stator unit can power rotation of the bacterial flagellar motor in both directions, which is crucial for the bacteria to change their swimming direction. Without direction change, bacteria would only be able to swim straight in one direction."

Next step for the group is to find out if it is possible to inhibit the stator units using chemical compounds, which could have antibiotic effects.

https://healthsciences.ku.dk/newsfaculty-news/2020/09/tiny-protein-motor-fuels-bacterial-movement/

https://www.cell.com/cell/fulltext/S0092-8674(20)31007-2