The sensory regulation of virulence in Vibrio cholerae

Cholera is a fatal acute diarrhoeal disease, that can become deadly within hours if not treated properly. The importance of research in this field is highlighted by the number of infections (1.3 - 4.0 million) and caused deaths (21 000 -143 000) annually in 51 endemic countries. It is of great importance to provide affordable medication that is in reach for even resource -poor regions, which suffer the most from reoccurring cholera outbreaks. But not only developing countries are affected. The probability of an outbreak is connected to poor sanitation, therefore regions experiencing natural disasters like earthquakes, floods or droughts have also a heightened risk of a cholera outbreak. The rapid spreading of the disease among countries and even continents represents not only an incredible danger for the people due to its high morbidity, it is also a threat to the economies of the affected countries. The resistance of cholera causative Vibrio cholerae to common antibiotics, further emphasizes the need for the development of effective treatment. This project concentrates on the functional and structural characterization of regulatory proteins from Vibrio cholerae. The ability of the bacterium to rapidly adapt to changing environmental conditions (e.g. upon oral ingestion by human), enables its long-term environmental persistence. The interaction between regulatory proteins plays a fundamental role in this adaptive virulence system. One of the main goals of our research is therefore to characterize the interplay of these proteins, and subsequently find substances that inhibit the interaction. The gained information could form the basis for the development of new effective medication against the cholera disease. At the same time, we could provide crucial insights into the functionality of the bacterium and thereby provide valuable knowledge about Vibrio choleraes surviving mechanisms, which is essential for controlling the spreading of the disease.

The four-year FWF funding enabled us to identify a novel and highly promising drug target against cholera, a severe diarrheal disease that has been pandemic since 1961. The key lies in how the cholera bacterium Vibrio cholerae senses its environment inside the human body and induces its disease-causing program. We have uncovered that two proteins, ToxR and ToxS, form a molecular sensor complex that detects bile acids in the human intestine. Bile acid serves as a signal that the bacterium has entered a host environment. In our experiments, we resolved the atomic structure of the ToxR-ToxS protein complex and demonstrated how together they form a bile-binding pocket. This binding pocket is correctly shaped only when both proteins are joined: ToxS provides the binding cavity and ToxR stabilizes it and translates the signal into a change in gene activity. The binding of bile triggers ToxR to switch on the expression of virulence genes: the bacterium shifts from a dormant, low-activity mode into a highly active, pathogenic state. At the same time, bile recognition protects the bacterium itself, as the activation of ToxR-dependent genes also strengthens the bacterial surface against the damaging effects of bile. These insights into the dual role of bile sensing highlight ToxRS as a key regulator of both survival and virulence. Importantly, our data show that this sensor system is highly conserved among different Vibrio species, many of which are relevant to human and animal health. This means that therapeutic strategies developed against the ToxRS complex in V. cholerae could also be applicable to other pathogenic Vibrio bacteria. From a drug development perspective, the structural features of ToxRS are particularly attractive: the complex is located in the bacterial periplasm and therefore directly accessible to small molecules. It has no structural counterparts in humans, minimizing the risk of side effects. Furthermore, its role as a tightly regulated two-component system makes it especially suitable as a drug target. If successful, this "anti-virulence" strategy would interfere with the bacterium's ability to cause disease without necessarily killing it outright - a method that may reduce the evolutionary pressure to develop resistance compared to conventional antibiotics. Such approaches are urgently needed, as antibiotic resistance in cholera and other pathogens is a growing global health challenge. Our findings open up a new molecular window into how V. cholerae and related pathogens sense and adapt to their environment in the human body. By providing both the structural and functional understanding of this central sensory switch, we pave the way for innovative treatments against cholera and possibly other Vibrio-associated infections, with significant potential impact on global health.