Bacterial resistance across habitats: from German schools to the International Space Station.

Environmental factors play a crucial role in the emergence, persistence, and spread of bacterial resistance. The habitat of microorganisms can define their stress resistance profile. Through interaction with other microorganisms, the exchange of microbial resistance traits can be facilitated, thereby enhancing their overall resilience. Microorganisms exposed to space-related stressors, such as microgravity, increased levels of ionizing radiation, and desiccation, face distinctive survival challenges. These stressors can induce cellular adaptations that can affect microbial resistance. Moreover, confinement and cleaning routine can further drive the evolution of more resistant strains that could influence microbial communities in spacecraft habitats. Hence, assessing the resistance to multiple stressors is critical for understanding how different habitats shape resistance profiles and for developing microbial control strategies that may help ensure the safety of space missions. This is particularly important as humanity plans for prolonged space travel and habitation on extraterrestrial surfaces. This study explores the stress resistance of bacterial isolates collected from the International Space Station and German schools in the context of the spaceflight experiment "Touching Surfaces," which tested antimicrobial surfaces on the ISS and on Earth. The isolates were tested for their antibiotic resistance revealing no detectable increase in antibiotic resistance under the conditions tested in isolates from space or schools. Additionally, isolates of the skin commensal Micrococcus luteus and the opportunistic pathogens Staphylococcus haemolyticus, Staphylococcus epidermidis and a methicillin-resistant Staphylococcus aureus (MRSA) were tested for their tolerance to key environmental stressors relevant to spaceflight. Stress resistance assays included desiccation, X-ray irradiation, and hydrogen peroxide treatment. Overall, space isolates did not show an increased resistance to desiccation or, X-ray irradiation. However, both space and school isolates of Staphylococcus spp. recovered from surfaces showed increased survival after treatment with hydrogen peroxide compared to their respective type strains. Additionally, early-stage biofilm formation was significantly higher for space and school isolates of Micrococcus sp. compared to the type strain M. luteus.