Vibrio cholerae is "activated upon receiving light," increasing flagellar motility

A research group including Assistant Professor Jun Xu and Professor Emeritus Tetsu Yamashiro of the Graduate School of Medicine, University of the Ryukyus, Associate Professor Shuichi Nakamura of the Graduate School of Engineering, Tohoku University, and Researcher Rintaro Tomioka of the Graduate School of Engineering, the University of Tokyo announced on April 13 that they have discovered that Vibrio cholerae tends to move more actively when exposed to light. They found that a photo-activated adenylyl cyclase (PAC) in V. cholerae increases the signaling molecule cAMP, which in turn enhances flagellar motility. This mechanism may be involved in the early stages of environmental adaptation and the establishment of infection, and these findings are expected to lead to its elucidation. The results were published in PNAS on April 9.

Cholera is an acute infectious disease caused by ingesting water or food contaminated with the bacterium V. cholerae, and it remains a public health problem worldwide. Once colonized and multiplying in the large intestine, the bacterium produces cholera toxin, causing massive watery diarrhea.

The "power to move" is crucial for the bacterium's infectivity. It was previously known to swim through water by rotating a long, thin flagellum, altering its behavior in response to environmental changes such as temperature, nutrients, and chemical substances. On the other hand, although it inhabits aquatic environments where light reaches, its response to light remained unexplained.

Therefore, in this study, the research group investigated V. cholerae's response to light in detail. First, they examined its movement isolated from river water by exposing the bacteria to light while observing them under a microscope. They found that upon receiving light, their swimming speed increased, and the number of moving bacteria also grew.

When conducting an experiment that created bright and dark areas within the same field of view, a tendency for the bacteria's movement to become more active in the bright areas was observed.

A special enzyme that starts working upon receiving light (PAC) was involved in this reaction. PAC produces the intracellular signaling molecule "cAMP" inside the bacterial cell, which activates the sodium transport system.

It is thought that the acceleration of extracellular sodium ion extrusion increases the sodium concentration gradient, which expands the sodium motive force which serves as the rotational energy for the flagellar motor, thereby activating the motility of V. cholerae. This demonstrates that light is transmitted into the cell as "information," and the bacterium adjusts its own motility in accordance with that signal.

This indicates that pathogenic bacteria skillfully read their surrounding environment, and further investigation into this environmental response mechanism could lead to the control of pathogenic bacterial infections.

Xu stated: "In this study, we clarified the mechanism by which Vibrio cholerae regulates its motility using the environmental information of light. I believe it is important that bacteria are shown not to be merely affected by the environment, but to take it in as information and change their behavior. In the future, by deepening our understanding of such environmental responses, I hope to connect this to the elucidation of the behavior and survival strategies of pathogenic bacteria."

Journal Information
Publication: PNAS
Title: Light-activated cAMP signaling controls sodium-driven motility in Vibrio cholerae
DOI: 10.1073/pnas.2530860123