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Extracellular ATP (eATP) is increasingly recognized as a key regulator of stress, damage and protective responses across biological systems. While its signalling role in mammalian cells is well established, the mechanisms governing its function and regulation in the context of host-microbe interactions remain incompletely understood. This thesis explores how eATP levels are sensed and modulated by bacteria, and how its dysregulation impacts host physiology during infection. We first demonstrate that E. coli and related Gammaproteobacteria possess enzymatic pathways capable of degrading eATP to hypoxanthine, thereby actively modulating eATP concentrations in the intestinal environment. Through genome-wide screening and biochemical validation, we identified key bacterial factors involved in this degradation process and confirmed their functional relevance in vitro, in enterocyte infection models, and in vivo. These findings reveal a previously underappreciated microbial strategy to modify intestinal inflammation by controlling eATP availability.

Further, we show that E. coli does not merely degrade eATP but also responds to it as an environmental signal. Gene expression and metabolomic analyses revealed that eATP exposure reshapes bacterial physiology, regulating genes involved in metabolism, stress responses, antimicrobial resistance and virulence. This dual role of eATP as both a substrate and a signal highlights its importance in shaping microbial behavior and host- microbe interactions.

Extending the scope beyond bacterial systems, we investigated systemic eATP dynamics in the context of viral infection in a longitudinal study of 394 COVID-19 patients. Plasma eATP levels were elevated during acute SARS-CoV-2 infection and remained dysregulated for up to a year, independently of disease severity. High eATP correlated with markers of coagulation, kidney function and chronic immune activation. Notably, eATP peaks were associated with the development of humoral immunity during acute infection, and were further elevated following COVID-19 vaccination, suggesting a role for eATP in shaping long-term immune trajectories.

Together, this thesis presents a unified view of eATP as a central mediator connecting microbial processes, host immune signalling, and systemic inflammation. By integrating bacterial and viral contexts, it advances our understanding of how eATP contributes to health and disease, and opens new avenues for therapeutic strategies targeting ATP- dependent signalling.