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This study addresses the microbial risks associated with Underground Hydrogen Storage (UHS), a critical component in the transition towards renewable energy systems, by employing an innovative multi-reactor system (Bio-xplorer) to simulate UHS conditions in two Italian reservoirs. The microbiological risk assessment (MRA) of Reservoir A and B was evaluated by subjecting them to gas mixtures of 10 % H 2 and 90 % CH 4 , and 99 % H 2 and 1 % CO 2 , respectively. In Reservoir A, the stability of pressure and temperature, the negligible optical density, and lack of microbial metabolites suggested a low risk of microbial activation. Molecular analyses confirmed the absence of sulphate-reducing bacteria (SRB) and limited growth of hydrogenotrophic methanogens (HM). Similarly, in Reservoir B, the absence of SRB and limited occurrence of HM indicated a low microbiological risk. Overall, the present work supports the safe and efficient implementation of UHS , a promising mitigation technique for climate change, using an innovative tool for MRA. [Display omitted] • Microbial risk assessment for UHS in Italy. • Innovative HP & HT bioreactors system (Bio-xplorer) for UHS lab tests. • Novel approach for UHS Microbial risk assessment in two Italian NG reservoirs. • Reactor-based assessment of SRB, HM, AB activity by mimicking reservoirs environment. • No sensible hydrogen consumption registered for both the reservoirs. [ABSTRACT FROM AUTHOR]