Scientists have uncovered a groundbreaking explanation for how swimming bacteria change direction, offering fresh insight into one of biology’s most intensively studied molecular machines. This discovery, published in Nature Physics by researchers from the Flatiron Institute, challenges long-held beliefs about the mechanics of bacterial movement.
Bacteria navigate through liquids using propellerlike tails known as flagella, which alternate between clockwise and counterclockwise rotations. For decades, this behavior was attributed to an equilibrium ‘domino effect’ model, where proteins lining the bacterium’s tail exert pressure on their neighbors, prompting a change in direction. However, the new research by Henry Mattingly and Yuhai Tu proposes a different mechanism: an active tug-of-war among distant proteins.
The Problem With the Domino Effect
The flagellar motor, a marvel of nature’s engineering, consists of 34 proteins arranged in a central ring, powered by structures called stators. These stators allow electrically charged atoms to flow in and drive the motor’s rotation. The ring proteins determine the rotational direction based on signals from a molecule called CheY-P. When CheY-P binds to a protein, it influences the protein’s conformation, promoting rotation in a specific direction.
According to Mattingly, CheY-P acts as a relay from the cell’s environment to its behavioral response. Depending on which proteins are bound by CheY-P, the motor may favor clockwise or counterclockwise rotation. The original equilibrium model suggested that neighboring proteins would eventually align through a domino effect. However, experimental data showed deviations from this model, indicating that the motor’s switching behavior was not purely equilibrium-based.
“If you see this pattern, then the effect cannot be a purely equilibrium phenomenon,” says Tu. “There had to be something else going on.”
A Tug-of-War Inside the Tail
Mattingly and Tu’s research suggests that the motor’s directional switch is not passive but involves energy injection into the system. Recent discoveries about the motor’s structure informed their theory. The C-ring, a large central gear, interacts with stators that function as smaller gears. These stators always rotate clockwise, influencing the motor’s direction based on how they contact the C-ring’s teeth.
Conflicts arise when different teeth adopt opposing conformations, leading to a mechanical tug-of-war. This process, termed “global mechanical coupling,” highlights that the forces driving each tooth are not solely determined by neighboring interactions but involve a collective process across the motor.
“Global mechanical coupling explains what the earlier, purely equilibrium theory couldn’t, that switching is energy-driven, directional and cooperative,” says Tu.
Unraveling Mysteries in the Flagella and Beyond
This new model not only advances our understanding of bacterial movement but also provides insights into other nonequilibrium systems in living organisms. Tu emphasizes that living systems operate out of equilibrium, dissipating energy essential for biological functions. The flagellar motor serves as a perfect system for testing ideas, potentially informing our understanding of more complex biological systems.
Mattingly and Tu plan to refine their model with additional experimental data, noting that their predictions align with observed patterns, albeit on different timescales. The implications extend beyond bacterial movement, offering new perspectives in the field of bacterial chemotaxis.
“Every so often people say, ‘This is a dead field.’ Every time that turns out to be wrong. There’s always another layer,” Tu concludes.
As research continues, these findings may lead to broader applications in understanding biological systems, reaffirming the dynamic and ever-evolving nature of scientific inquiry.
