Impact of Various Inactivation Approaches on Surrogate Proteinaceous Particles for Sample Return Missions

Return of Mars samples is a high priority in the planetary science community and has remained an enduring goal of planetary exploration programs. Development of sterilization techniques to prevent potential contamination of Earth's biosphere with unknown life-forms that could exist on planetary bodies requires the use of the most robust biological indicators. We argue that self-seeding proteinaceous particles (prions) represent the most robust biological agents found on Earth. To evaluate the impact of various sterilization techniques on prion activity, we used derivatives of yeast prion proteins Sup35 and Ure2, which are not harmful to humans. Our study demonstrated that effective antimicrobial modalities, which include prolonged dry heat (up to 200°C), vapor hydrogen peroxide, gamma irradiation (up to 100 kGy), and ambient air or wet He/water plasma (deposited energy density of up to 6.3 kJ/cm²), did not eliminate the biological activity of yeast prions. However, ultraviolet C (UVC) irradiation at a wavelength of 260-270 nm for 16-24 days eliminated Ure2 prion detection and biological activity, and prolonged UVC irradiation eliminated detection of Sup35 prions and reduced, although did not eliminate, their biological activity. These data suggest that UVC could be an essential component of in-flight sterilization techniques for all future planetary missions.