Survival of spacecraft-associated microorganisms under simulated martian UV irradiation

Abstract

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.

FIG. 1.

Spectral irradiance plots for the JPL solar simulator (dashed line) and the KSC simulator (solid line). Also included for reference are spectral irradiance plots for the Opportunity landing site under a high sun angle and sky conditions producing optical depths of 0.24 (◊) and 0.74 (□) and irradiance plots predicted for latitude 45°S under a low sun angle and optical depths of 0.24 (○) and 0.74 (▵).

FIG. 2.

Phylogenetic tree of the type strains of Bacillus spp. used in this study based on maximum-likelihood parsimony analysis of the 16S rRNA gene nucleotide sequences. The numbers after the names of the bacteria are GenBank nucleotide sequence accession numbers. The numbers above the lines are the bootstrap percentages for 500 replications of the branches of the tree.

FIG. 3.

Effect of Mars UV irradiation (JPL simulator) on hydrated spores of select Bacillus species. ▪, B. pumilus SAFR-032; •, B. megaterium ATCC 14581; ▿, B. subtilis 168. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

FIG. 4.

Influence of various Mars UV spectra on the survival of B. pumilus SAFR-032 spores in water (JPL simulator). •, UVA; ▿, UVA+B; ▪, full UV spectrum. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

FIG. 5.

Effect of B. pumilus SAFR-032 on spores susceptible to UV-induced inactivation (JPL simulator). ▪, B. subtilis 168 in mixture; ▿, B. subtilis 168 alone; •, B. pumilus SAFR-032 alone. In the mixed samples equal amounts of spores from the two species were mixed to obtain a total spore density of 10⁶ spores ml⁻¹. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

FIG. 6.

Effect of UV₂₅₄ irradiation on vegetative cells. ▪, B. pumilus SAFR-032; •, A. radioresistens; ▾, B. subtilis 168. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

FIG. 7.

Effect of Mars UV irradiation, projected from the KSC simulator, on the spores of B. pumilus SAFR-032 (▪) and B. subtilis 168 (▪) in water. (A) Full UV spectrum; (B) UVA+B; (C) UVA. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.