Scientists have uncovered a groundbreaking explanation for how swimming bacteria change direction, shedding light on one of biology’s most intensely studied molecular machines. This discovery, published in Nature Physics, challenges long-standing beliefs about bacterial movement and could have far-reaching implications for understanding biological systems.
Bacteria navigate through liquids using propeller-like structures known as flagella. These flagella switch between clockwise and counterclockwise rotations, a behavior previously attributed to a ‘domino effect’ model. This model suggested that proteins lining the bacterium’s tail exert pressure on their neighbors, prompting a change in rotational direction.
However, new research conducted by Henry Mattingly and Yuhai Tu from the Flatiron Institute proposes a different mechanism. Their findings, informed by experimental measurements of the flagellar motor’s molecular structure, suggest that the switch is driven by an active tug-of-war among distant proteins.
The Problem With the Domino Effect
The flagellar motor, a structure long admired for its complexity, comprises 34 proteins arranged in a central ring. This ring is powered by smaller structures called stators, which allow electrically charged atoms to flow in and drive rotation. The proteins in this ring control the direction of rotation based on signals from a molecule called CheY-P.
“CheY-P concentration depends on what the cell is experiencing outside, in its environment,” explains Mattingly, an associate research scientist at the Flatiron Institute’s Center for Computational Biology (CCB). “It’s like a relay from what the cell senses to how it responds with changes in behavior.”
In the original equilibrium model, it was believed that neighboring proteins would eventually align through a domino effect. However, experimental data revealed inconsistencies with this model, showing a peak in the time distribution for rotation that couldn’t be explained by equilibrium theories.
A Tug-of-War Inside the Tail
Mattingly and Tu’s research suggests that energy is injected into the system, influencing how and when the motor switches direction. Recent discoveries about the motor’s physical structure informed their theory, particularly the role of the C-ring and stators as gears in this process.
Conflicts arise when different teeth in the C-ring adopt opposing conformations, leading to a mechanical tug-of-war. Mattingly describes the process: “Imagine all the teeth are in the same outer conformation. Then one of them flips. As the gear turns, that lone dissenter eventually comes in contact with a stator that now pushes it in the opposite direction from all the others.”
This tug-of-war, termed “global mechanical coupling,” explains the observed peak in rotational direction switching, highlighting the active role of stators in driving the system out of equilibrium.
Unraveling Mysteries in the Flagella and Beyond
The implications of this study extend beyond understanding bacterial movement. According to Tu, “Our results make sense to me because I believe living systems always operate out of equilibrium. They dissipate energy, and that energy is essential for biological function.”
The team plans to refine their model further, integrating more experimental data to better predict the distribution of rotational durations. Understanding flagella could also influence the study of more complex biological systems.
“It’s so well studied that it becomes a perfect system to test ideas — and what we learn here often helps us think about more complex biology,” says Mattingly.
The research also holds promise for the field of bacterial chemotaxis. Tu notes, “Every so often people say, ‘This is a dead field.’ Every time that turns out to be wrong. There’s always another layer.”
As the scientific community continues to explore the intricacies of bacterial movement, this new model offers a fresh perspective and underscores the dynamic nature of biological research.
