Resistance of Bacterial Endospores to Outer Space for Planetary Protection Purposes—Experiment PROTECT of the EXPOSE-E Mission

Abstract

Spore-forming bacteria are of particular concern in the context of planetary protection because their tough endospores may withstand certain sterilization procedures as well as the harsh environments of outer space or planetary surfaces. To test their hardiness on a hypothetical mission to Mars, spores of Bacillus subtilis 168 and Bacillus pumilus SAFR-032 were exposed for 1.5 years to selected parameters of space in the experiment PRO-TECT during the EXPOSE-E mission on board the International Space Station. Mounted as dry layers on spacecraft-qualified aluminum coupons, the ''trip to Mars'' spores experienced space vacuum, cosmic and extraterrestrial solar radiation, and temperature fluctuations, whereas the ''stay on Mars'' spores were subjected to a simulated martian environment that included atmospheric pressure and composition, and UV and cosmic radiation. The survival of spores from both assays was determined after retrieval. It was clearly shown that solar extraterrestrial UV radiation (k ‡ 110 nm) as well as the martian UV spectrum (k ‡ 200 nm) was the most deleterious factor applied; in some samples only a few survivors were recovered from spores exposed in monolayers. Spores in multilayers survived better by several orders of magnitude. All other environmental parameters encountered by the ''trip to Mars'' or ''stay on Mars'' spores did little harm to the spores, which showed about 50% survival or more. The data demonstrate the high chance of survival of spores on a Mars mission, if protected against solar irradiation. These results will have implications for planetary protection considerations.

The SPORES experiment of the EXPOSE-R mission:Bacillus subtilisspores in artificial meteorites

“…Neither replacing space vacuum by an Ar atmosphere nor mixing the spores with MSR07 powder altered those results (Table 3). They confirm earlier observations on the extreme lethality of extraterrestrial solar UV radiation (Nicholson et al 2000;Horneck et al 2010Horneck et al , 2012. ]; however, for spores UV irradiated in space vacuum, mixing with MRS07 powder did not result in any survivors.…”

Surviving in hot and cold: psychrophiles and thermophiles from Deception Island volcano, Antarctica

“…Bacterial spores are well-established models for astrobiology, due to their resistance to multiple factors, including desiccation and UV irradiation. Spores of Bacillus pumilus and Bacillus subtilis, for example, have already been exposed and tested for their survival under true space conditions (Horneck et al 2012;Panitz et al 2015). Recently, Khodadad et al (2017) directly exposed desiccated spores of B. pumilus to the stratosphere (above 30 km of altitude).…”

Study of the effects of the outer space environment on dormant forms of microorganisms, fungi and plants in the ‘Expose-R’ experiment

“…Exteriors of human and robotic space vehicles are inhabited by millions of microbial cells, many of which are carried as spores that are highly resistant to the adverse environmental effects (La Duc et al 2003). Their resistance to outer space conditions needs to be verified by specifically designed experiments, as already independently demonstrated in several successful experiments (for review see Horneck et al 2010Horneck et al , 2012Tepfer et al 2012;Vaishampayan et al 2012;Wassmann et al 2012).…”