The model organism used in this study—Yersinia pseudotuberculosis, a close relative of the infamous plague pathogen—is a true master of temperature sensing. It uses environmental temperature as a signal to distinguish between the cool external environment and the warm conditions inside a host organism.

It was already known that RNA molecules can directly sense temperature, but the new study has shown that the Fis protein also plays a regulatory role at the DNA level. “Fis acts as a molecular regulator that suppresses virulence genes (the degree of pathogenicity) at 25°C outside the host,” explained Professor Franz Narberhaus from the Department of Microbiology at Ruhr University Bochum.

Once inside a warm-blooded host, two processes occur: the production of proteins responsible for bacterial motility is suppressed to avoid detection by the human immune system, while virulence factors that weaken the body’s defenses are activated.

“In experiments with a Yersinia strain lacking the Fis protein, we observed an unexpected phenotype at 25°C,” said Narberhaus. “The bacteria were completely immobile and secreted effector proteins that typically exert harmful effects only in the intestine.” These findings indicate that the absence of Fis disrupts the temperature-dependent balance between non-pathogenic and pathogenic states.

The results raise the question of whether bacteria lacking the Fis protein may be infectious even at 25°C. Virulence is usually studied in mice, which, like humans, are warm-blooded. However, such models are not suitable for experiments involving different temperature conditions.

A more appropriate alternative proved to be moth larvae, which are poikilothermic organisms (their body temperature varies with the environment). The Yersinia strain lacking the Fis protein showed increased virulence at 25°C. Unlike the wild-type strain, it was capable of killing larvae even at this relatively low temperature.

This suggests that the Fis protein suppresses virulence outside the host organism. Researchers in Bochum are now working to determine the precise molecular mechanisms by which Fis exerts this regulatory control.