Spore-forming microbes recovered from spacecraft surfaces and assembly facilities were exposed to simulated Martian UV irradiation. The effects of UVA (315 to 400 nm), UVA؉B (280 to 400 nm), and the full UV spectrum (200 to 400 nm) on the survival of microorganisms were studied at UV intensities expected to strike the surfaces of Mars. Microbial species isolated from the surfaces of several spacecraft, including Mars Odyssey, X-2000 (avionics), and the International Space Station, and their assembly facilities were identified using 16S rRNA gene sequencing. Forty-three Bacillus spore lines were screened, and 19 isolates showed resistance to UVC irradiation (200 to 280 nm) after exposure to 1,000 J m ؊2 of UVC irradiation at 254 nm using a low-pressure mercury lamp. Spores of Bacillus species isolated from spacecraft-associated surfaces were more resistant than a standard dosimetric strain, Bacillus subtilis 168. In addition, the exposure time required for UVA؉B irradiation to reduce the viable spore numbers by 90% was 35-fold longer than the exposure time required for the full UV spectrum to do this, confirming that UVC is the primary biocidal bandwidth. Among the Bacillus species tested, spores of a Bacillus pumilus strain showed the greatest resistance to all three UV bandwidths, as well as the total spectrum. The resistance to simulated Mars UV irradiation was strain specific; B. pumilus SAFR-032 exhibited greater resistance than all other strains tested. The isolation of organisms like B. pumilus SAFR-032 and the greater survival of this organism (sixfold) than of the standard dosimetric strains should be considered when the sanitation capabilities of UV irradiation are determined.When vegetative cells of Bacillus species are confronted with low nutrient abundance, the bacteria can initiate the process of sporulation, in which the growing cells differentiate into dormant spores (48). Spores do not metabolize at a detectable level and are highly resistant to several perturbations, such as heat and exposure to UV and gamma radiation (33). Due to this intrinsic resistance spores are ubiquitous in the environment and have been found both above and below the surface of the Earth (33, 38).The resistance of spores has prompted agencies such as the National Aeronautics and Space Administration (NASA), the Department of Homeland Security, and others to study sporulating bacteria more closely (1,2,9,11,15,36,43,50). Recently, in several microbial diversity surveys performed over a period of 3 years, 125 aerobic microbial strains were isolated from spacecraft assembly facilities (20,24,25,52,53), and their phylogenetic affiliations were determined (23, 54). Eighty-five percent of these strains were identified as gram-positive bacteria. About 65% of the strains cultivated survived heat shock protocols used to isolate sporulating bacteria (2). Members of the genus Bacillus were the predominant microbes among the heat shock survivors (Ͼ91%). A total of 15 different Bacillus species were identified. Bacillus licheniformis ...
