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Applied and Environmental Microbiology, December 2005, p. 8147-8156, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8147-8156.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Survival of Spacecraft-Associated Microorganisms under Simulated Martian UV Irradiation

David A. Newcombe,1 Andrew C. Schuerger,2 James N. Benardini,1 Danielle Dickinson,1 Roger Tanner,3 and Kasthuri Venkateswaran1*

Biotechnology and Planetary Protection Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109,1 Department of Plant Pathology, University of Florida, Space Life Sciences Lab, Kennedy Space Center, Florida 32899,2 Department of Planetary Science, University of Arizona, Space Sciences Bldg., Tucson, Arizona 857213

Received 13 January 2005/ Accepted 2 August 2005


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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.


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INTRODUCTION
 
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 (>91%). A total of 15 different Bacillus species were identified. Bacillus licheniformis was the most prevalent species (25%) and Bacillus pumilus (16%) was the second-most prevalent species isolated from spacecraft assembly facilities.

Dormant spores of various Bacillus and Clostridium species are much more resistant than their vegetative cell counterparts to a variety of treatments, including UV irradiation (49). UV irradiation has been used as a sporicide to decontaminate or sterilize surfaces, air, and water (5, 19, 40, 56). Previous work has demonstrated that binding of {alpha}/ß-type small acid-soluble proteins to spore DNA is the predominant if not sole determinant of spore UV resistance (47). Further studies using Bacillus subtilis spores implicated several genes, including dacB, spl, and ssp, as genes that are essential in UV irradiation resistance (37). In addition, the spore coat has been shown to provide intrinsic protection against UVA radiation (315 to 400 nm) and UVB radiation (280 to 315 nm) (39). However, there have been few studies that have tested the UV resistance of a diverse range of environmental strains of Bacillus species (28). Invariably, all microbial lethality assessments and sterilizer validation studies have been carried out using laboratory strains of B. subtilis and/or Geobacillus stearothermophilus (32). Microbes inhabiting nutrient-poor and dry spacecraft assembly facilities and the resistance traits of these organisms are important to the aerospace, medical, and pharmaceutical industries. There has been growing concern that the hardy nature of spores could allow them to escape their earthly confines, survive in the vacuum of space, and intercept extraterrestrial bodies (12, 29). In addition to these concerns, the resistance of spores could enable the organisms to escape sterilization protocols and possibly survive aboard spacecraft to contaminate planets or contaminate critical industrial implements.

In this study, we screened endospores of isolated spore-forming bacteria from a growing collection of spacecraft and associated environmental isolates for resistance to UV irradiation at 254 nm (UV254). In order to examine the biocidal effects of direct UV irradiation predicted for Mars, UV254-resistant spores were exposed to simulated Martian UV irradiation. The effects of UVA, UVA+B (280 to 400 nm), UVC (200 to 280 nm), and total UV irradiation (200 to 400 nm) on the survival of microorganisms at intensities expected to strike the surface of equatorial Mars were examined.


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MATERIALS AND METHODS
 
Isolation and identification of microbes from spacecraft surfaces and associated assembly facilities.
Species of Bacillus were previously isolated from surfaces of the Mars Odyssey and X-2000 (avionics) spacecraft and their associated assembly facilities at the Jet Propulsion Laboratory (JPL), Kennedy Space Center (KSC), and BAE Systems (Manassas, VA). Bacillus species were also isolated from surface samples taken in the International Space Station. Surface areas of approximately 25 cm2 were sampled using water-moistened, sterile polyester swabs (catalog no. TX761; Texwipe). Selected floor surfaces with areas of approximately 3,600 cm2 were sampled using sterile wipes (catalog no. TX1009; Texwipe). The swab and wipe samples were placed into sterile polypropylene bottles containing 10 and 100 ml sterile water, respectively, and transferred to the laboratory for analysis. Swabs exposed to the assembly facility atmosphere but not used in active sample collection served as controls. The spore-forming bacterial populations were selected by sonicating the samples for 2 min and heat shocking them at 80°C for 15 min. Appropriate aliquots of samples were placed into petri dishes in duplicate using the pour plate technique with Trypticase soy agar (TSA) (Becton Dickinson and Co.) as the growth medium. Following 2 days of incubation at 32°C (1, 2) isolates were selected, purified, and stored at –80°C for further processing and analysis. All other strains were purchased from the American Type Culture Collection and are listed in Table 1.


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  		TABLE 1. Characteristics of microbes associated with several spacecraft and associated environments tested in this study

The identities of purified strains were determined by using 16S rRNA gene sequencing. Bacterial small-subunit rRNA genes were PCR amplified with eubacterially biased primers B27F and B1492R (26). PCR conditions described elsewhere were used (41). The PCR-amplified 16S rRNA gene fragments were purified using Qiaquick columns (QIAGEN) and were fully, bidirectionally sequenced. The phylogenetic relationships of organisms covered in this study were determined by comparison of individual 16S rRNA gene sequences to sequences in the public database described at http://www.ncbi.nlm.nih.gov. Evolutionary trees were constructed by phylogenetic analysis using parsimony software described at http://paup.csit.fsu.edu. The sources of the strains tested and their 16S rRNA gene sequences and GenBank accession numbers are shown in Table 1.

Sporulation.
A nutrient broth sporulation medium was used for the initial sporulation of test bacteria (30, 45). A single purified colony was inoculated into liquid nutrient broth sporulation medium and incubated at 32°C, and the cultures were examined by using wet mounts and light microscopy each day to determine the level of sporulation. Once the number of free spores in each culture was greater than 99% of the total number of cells present, typically 2 to 3 days, cultures were harvested by centrifugation. Spore purification was performed by treating the spores with lysozyme and washing them with salts and detergent, as described by Nicholson and Setlow (30). Purified spores were resuspended in sterile deionized water, heat shocked at 80°C for 15 min, and stored at 4°C in glass tubes until they were used.

Selection of UV254-resistant spores.
Purified spores of 44 strains (Table 1) were diluted in phosphate-buffered saline (pH 7.2) to obtain a density of 106 spores per ml. The initial spore density was estimated by the dilution plating method before each exposure. A low-pressure handheld mercury arc UV lamp (model UVG-11; UVP, Inc.) was placed over a sample, and the UV flux at the surface of the spore suspension was measured using a UVX digital radiometer (UVP, Inc.). The exposure time required to produce 1,000 J m–2 of energy at the sample surface was determined to be 167 s at 600 µW cm–2. The spore suspension was placed in an uncovered 100-mm glass petri dish containing a magnetic stir bar and was exposed to UV254 irradiation under sterile conditions. In a qualitative screening analysis, strains surviving 1,000 J m–2 irradiation were selected for quantitative lethal dose curve analysis. Samples (100 µl) were removed after specific periods of time, serially diluted, and plated on TSA. The dose at which 90% of the spores were inactivated (LD90) was >200 J m–2 were considered resistant and were used for further experiments.

Mars atmospheric radiation model.
Figure 1 compares the output of a Mars atmospheric irradiation model with the spectral irradiances used in the lab experiments. The Mars atmosphere UV model used in this study was developed using Mars Pathfinder data for visible light to near-infrared light and atmospheric transmittance models developed at the University of Arizona Lunar and Planetary Lab (51). Expansion of the model into the UV bandwidth was accomplished by extrapolation and by comparing the results at specific wavelengths to observational data for Mars dust-like materials (unpublished data). Four conditions were modeled to explore the range of UV irradiance for a likely lander site (Fig. 1). Two levels of dust were modeled (optical densities at 671 nm of 0.24 and 0.74), and these levels correspond to the range of dust levels experienced by the Opportunity rover (4). The high-sun conditions were typical for a lander with the sun directly overhead, a solar zenith angle of 2.06°. The low-sun conditions simulated a position 45° south during the Martian winter, with the same two optical densities of dust. Here the solar zenith angle was 70.2°, which significantly reduced the UV irradiance due to dust absorption. The spectral shapes are very similar for the four model spectra since the shape of a spectrum is affected by dust absorption and scattering. Ozone absorption or ice scattering was not modeled as these conditions are not the most common conditions.



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  		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 ({lozenge}) and 0.74 ({square}) and irradiance plots predicted for latitude 45°S under a low sun angle and optical depths of 0.24 ({circ}) and 0.74 ({triangleup}).

Resistance of spores in water to simulated Mars UV irradiation.
Spores resistant to UV254 irradiation were exposed to UVA (315 to 400 nm), UVA+B (280 to 400 nm), or total UV (200 to 400 nm) at a simulated Mars constant level of total solar irradiance of 590 W m–2 (190 nm to 3 µm) (3) using an X-25 solar simulator (Spectrolab Inc.) equipped with a xenon arc lamp located at the Environmental Test Laboratory, JPL. The intensity of UV irradiation was approximately 10% of the intensity of the full spectrum (59.0 W m–2). The total level of irradiance in these simulations matched the actual Mars constant level of solar irradiance, 590 W m–2 (3). The intensity of the irradiation was constantly adjusted by fine-tuning the lamp power to achieve ±5% variability, as indicated by an Optronics Laboratories OL754 spectroradiometer. The various bandwidths were generated by using Corning glass (UVA) or plastic petri dish lids (UVA+B; Fisher Scientific) as filters in front of the sample cuvettes. The lethal doses were calculated for each strain in order to estimate LD50, LD90, and LD100 UV dosages. LD100 was defined as the UV dose at which no cultivable spores were recovered using the procedures described here.

Spores of nine strains that exhibited the highest LD90 were diluted with sterile deionized water to obtain approximately 1 x 106 spores ml–1. Then 2-ml aliquots of the suspensions were placed in 3-ml Suprasil quartz cuvettes (path length, 10 mm) equipped with a micro stir bar (3 by 3 mm; Fisher Scientific). When two strains were mixed for exposure, 5 x 105 spores ml–1 of each strain were mixed to obtain a final density of 106 spores ml–1. The UV exposure times ranged from 30 s to 30 min. At various intervals, 100-µl samples were removed, diluted serially 10-fold in sterile phosphate-buffered saline, and plated onto TSA. All TSA plates were incubated at 32°C for 24 to 48 h, and CFU were enumerated. The most resistant spores were selected for further experimentation. Quartz cuvettes were cleaned after each exposure experiment by rinsing them three times with 70% ethanol and then three times with 95% ethanol. The rinsed and dried cuvettes were placed in appropriate gas-permeable envelopes (Tyvek pouches; Advanced Sterilization Products) and sterilized by exposure to one to four cycles of hydrogen peroxide injection in a Sterrad 100S vapor hydrogen peroxide sterilizer (Advanced Sterilization Products).

In addition to the X-25 system spores of two strains, B. subtilis 168 and B. pumilus SAFR-032, were exposed to simulated Martian irradiation using a xenon arc lamp (model 6262; Oriel Instruments) located at KSC under spectral conditions reported previously (46). Various bandwidths were generated as described above. In all UV assays in which we examined a mixture of B. subtilis 168 and B. pumilus SAFR-032 enumeration was possible due to the distinctly different colony morphologies of the two organisms while they were growing on agar medium.


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RESULTS
 
Identification of UV irradiation-resistant microbes.
Initially, samples from several spacecraft surfaces and associated assembly facilities were subjected to a heat shock protocol (2) to isolate sporulating bacteria. Spores of 43 strains (Table 1) were screened for UV254 resistance using an Hg lamp. Nineteen strains exhibited growth after receiving a dose of 1,000 J m–2. Phylogenetic analyses were performed for all the strains tested, and the strains were unambiguously determined to be low-G+C-content gram-positive Firmicutes based on 16S rRNA gene sequence analysis. The 16S rRNA gene sequences of all isolates were compared, and a bootstrap analysis (500 replicates) was performed to avoid sampling artifacts. The analyses indicated that the strains tested exhibited close phylogenic relationships with Bacillus species. Neighbor-joining, parsimony, and maximum-likelihood analyses were performed with this subset of bacteria using several subdomains of the 16S rRNA genes. A maximum-likelihood phylogenetic tree based on 16S rRNA gene sequences of several Bacillus species is shown in Fig. 2. The branching order of this tree showed that there were two distinct clusters. The top clade consisted of Bacillus rRNA group 1 species, and the other clade was formed by species belonging to rRNA group 2, as well as the rRNA group 1 Bacillus species that produce exosporium-like structures (38).



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  		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.

The levels of 16S rRNA gene nucleotide sequence similarity between the strains tested in this study and closely related Bacillus species, recognized by GenBank "BLAST" searches, were between 91.6 and 99.8%. Sequence variation of <3% was observed between B. subtilis, B. mojavensis, B. atrophaeus, B. licheniformis, and B. pumilus (Fig. 2). The strains of B. subtilis and B. pumilus sequenced in this study were tightly bound phylogenetically, as all of them exhibited levels of 16S rRNA gene sequence similarity of >97.5%. Additionally, a high level of 16S rRNA gene sequence variation (~8%) was observed between members of rRNA group 1 and members of rRNA group 2; however, a high degree of dissimilarity within a well-described genus is not uncommon. Based on 16S rRNA analysis, five of the environmental strains tested, B. pumilus FO-33, FO-36b, SAFN-036, and SAFR-032 and B. subtilis 42HS-1, were identified as members of rRNA group 1, and two strains, B. odysseyi and B. psychrodurans VSE1-06, were identified as members of rRNA group 2 (Table 1).

Martian UV irradiation simulation.
The JPL and KSC lab spectra were very similar to the models in terms of shape and level, and the KSC spectrum was more representative (Fig. 1). The JPL spectrum was deficient in 250-nm energy and exhibited a significant dip at 330 nm. This resulted in lower energies in the UVC and UVA bands and very similar levels for the UVB band. The 330-nm dip seen in the JPL spectrum was most likely due to lens coatings in the solar simulator but was not deemed to be critical because most of the absorption band was in the UVA region of the solar simulation. Our results (see below) indicated that UVA had a minimal effect on the survival of bacterial spores. The JPL and KSC simulations of Mars solar environments were calibrated to fall within a range of UV irradiation encountered under clear-sky conditions, with an optical depth of <1, for the Mars rover Opportunity at high and low sun angles.

Survival of endospores in aqueous solution under simulated Martian UV irradiation conditions (JPL simulation).
Of the 19 strains that exhibited UVC resistance, seven spacecraft-associated isolates were chosen for further study due to elevated LD90 of UV irradiation (data not shown) or other traits. The bacterial strains chosen and the determining factors used for selection for exposure to simulated Mars UV irradiation were as follows: B. odysseyi was selected because of its morphological novelty (25); B. psychrodurans VSE1-06 was selected because of its low-temperature tolerance; four strains of B. pumilus were selected because of their predominant occurrence; and B. subtilis 42HS-1 was selected because it is a close relative of the well-studied reference strain B. subtilis 168 (6, 14). In addition to the environmental strains mentioned above, B. subtilis 168 was selected as a control since this strain has been used in numerous other resistance studies that have been described previously (13, 14, 16, 17, 33, 46). Similarly, B. megaterium ATCC 14581 was also chosen for further study due to its high UV resistance compared to the UV resistance of other reference strains used in the current study. Most of the B. pumilus environmental isolates were obtained from the JPL spacecraft assembly facility class 100K cleanrooms; the only exception was B. pumilus 015342-2 ISS, which was isolated from surfaces of the International Space Station. Both B. subtilis 42HS-1 and B. odysseyi were cultured from the surface of the Mars Odyssey, and B. psychrodurans VSE1-06 was recovered from air samples collected in the assembly facility for the Mars Exploration Rovers, Payload Hazardous Servicing Facility, KSC (Table 1).

The results of exposing Bacillus spores in aqueous solution to UVA, UVA+B, and total UV are shown in Table 2. Compared to the exposure time for the full UV spectrum, 2- to 25-fold increases in the time of exposure to UVA or UVA+B were required to reduce the viable spore counts by 50%. Likewise, 90% reductions in viable spore numbers required 35- and 140-fold-greater times of exposure to UVA+B and UVA, respectively, than to the full UV spectrum. As shown in Table 2, none of the Bacillus species tested was completely eradicated even after 30 min of exposure to UVA+B irradiation or UVA (Table 2). The LD50s, LD90s, and LD100s of the nine types of bacterial spores tested with various UV spectra in the Mars solar simulation showed that UVA+B irradiation was significantly less lethal than full-spectrum UV irradiation; therefore, as expected, the 200- to 280-nm range is more damaging than longer wavelengths. Although all spores tested exhibited sensitivity to UVA+B, most damage by UVA+B might be attributed to UVB. This was further confirmed by the observation that all the spores tested except the B. pumilus FO-033 spores were resistant to UVA, with growth observed even after 30 min of exposure (Table 1).


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  		TABLE 2. Times necessary to reduce numbers of various Bacillus spores that are exposed to various UV radiation conditions at the Mars equatorial solar constant

Based on the Bacillus species tested, resistance to UV A, UVA+B, or full-spectrum UV irradiation was found to be strain specific. However, for Mars full UV spectra, three of four B. pumilus strains tested in this study exhibited LD50s of 40 to 80 s and LD90s of 100 to 270 s. Except for B. pumilus SAFR-032 and B. megaterium, spores of the Bacillus species tested exhibited an LD50, LD90, and LD100 of <24 s, <60 s, and <120 s, respectively, for Mars full UV spectra. Furthermore, B. pumilus SAFR-032 spores that showed resistance to 2,000 J m–2 UV254 (28) were not completely killed after 30 min when they were exposed to the full UV spectrum under simulated Mars intensities. In addition, SAFR-032 spores showed greater resistance (LD50 of 84 s and LD90 of 270 s) than the spores of the other strains to exposure to the full UV spectrum under the simulated Mars solar UV irradiation conditions. Reference strain B. subtilis 168 exhibited LD50, LD90, and LD100 for the full UV spectrum of 24 s, 42 s, and 72 s, respectively.

Plots of the survival rates for B. pumilus SAFR-032, B. megaterium, and B. subtilis 168 at various times for full Mars UV irradiation are shown in Fig. 3. B. pumilus SAFR-032 spores exhibited classical inactivation kinetics, with a characteristic "shoulder" extending to 2 min followed by strict exponential inactivation. However, B. subtilis 168 and B. megaterium spores exhibited a sharp decline in viability immediately after 30 s of UV exposure. After 10 min of UV exposure, the cultivability of B. megaterium spores was completely lost, while a portion of B. pumilus SAFR-032 spores survived.



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  		FIG. 3. Effect of Mars UV irradiation (JPL simulator) on hydrated spores of select Bacillus species. {blacksquare}, B. pumilus SAFR-032; •, B. megaterium ATCC 14581; {triangledown}, B. subtilis 168. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

Effects of various UV spectra under Mars solar UV irradiation conditions on the survival of B. pumilus SAFR-032 spores are shown in Fig. 4. Neither UVA nor UVA+B radiation was effective in inactivating SAFR-032 spores, even after 30 min of exposure, whereas the full simulated spectrum reduced SAFR-032 spore viability by more than 3 orders of magnitude after 30 min of irradiation. When the initial concentration of spores was reduced from 106 spores ml–1 to 105 spores ml–1 and to 104 spores ml–1 before UV exposure, the spores of B. pumilus SAFR-032 were completely killed in 8 and 2 min, respectively (data not shown).



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  		FIG. 4. Influence of various Mars UV spectra on the survival of B. pumilus SAFR-032 spores in water (JPL simulator). •, UVA; {triangledown}, UVA+B; {blacksquare}, full UV spectrum. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

The effects of viable and heat-killed SAFR-032 spores on the UV-sensitive B. subtilis 168 spores were tested (Fig. 5). Equal portions of viable spores of B. pumilus SAFR-032 and B. subtilis 168 were mixed to obtain a final density of approximately 106 spores ml–1 (5 x 105 spores ml–1 of each strain) and exposed to full simulated Martian UV irradiation. The time required to eradicate all B. subtilis 168 spores when they were mixed with an equal number of B. pumilus SAFR-032 spores was ~17 min, compared to 1.2 and 5 min when B. subtilis 168 spores at concentrations of 5 x 105 and 1 x 106 spores ml–1, respectively, were exposed alone (Fig. 3 and Table 2). Heat-killed B. pumilus SAFR-032 spores provided no protection to B. subtilis 168 (data not shown).



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  		FIG. 5. Effect of B. pumilus SAFR-032 on spores susceptible to UV-induced inactivation (JPL simulator). {blacksquare}, B. subtilis 168 in mixture; {triangledown}, 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.

Vegetative cells of B. pumilus SAFR-032, B. subtilis 168, and Acinetobacter radioresistens 50 v1, isolated from Mars Odyssey, were exposed to UV254 at a rate of 1 J m–2 s–1 and recovered under lighted conditions (Fig. 6). The results indicate that the three strains exhibited similar inactivation rates up to a total dose of 50 J m–2, after which cells of B. pumilus SAFR-032 and A. radioresistens survived a 200-J m–2 dose (a 3-log reduction), while B. subtilis 168 cells were unrecoverable after a 50-J m–2 dose.



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  		FIG. 6. Effect of UV254 irradiation on vegetative cells. {blacksquare}, B. pumilus SAFR-032; •, A. radioresistens; {blacktriangledown}, B. subtilis 168. The error bars indicate the standard deviations of three replicate samples. N/No, number of survivors/original number.

Survival of endospores in aqueous solutions under simulated Martian UV irradiation conditions (KSC simulation).
Since several publications have shown that there are variations in Martian UV simulations (7, 21, 46), we UV irradiated spores of B. pumilus SAFR-032 and B. pumilus 168 using an alternate Martian UV simulation system developed at KSC (32). Spores of these organisms were exposed to Martian UV, UVA, UVA+B, and full-spectrum irradiation conditions. Aliquots of UV-exposed spores were plated onto TSA after UV exposure for 0, 1, 2, 5, 10, and 20 min, and the surviving spore-forming cells were counted under appropriate cultural conditions. Figure 7A, B, and C show the inactivation curves for the spores of SAFR-032 and 168 exposed to full Mars UV, UVA+B, and UVA, respectively. The LD90 for B. subtilis 168 spores was <30 s when they were exposed to full Martian UV irradiation, and SAFR-032 spores exhibited at least threefold-higher resistance than 168 spores exhibited (Fig. 7A). Under the total UV spectrum the cultivability of B. subtilis 168 spores was lost after approximately 90 s, while B. pumilus SAFR-032 exhibited two- to threefold-greater resistance, remaining cultivable after 5 min (Fig. 7A). With UVA+B exposure, the survival of B. pumilus SAFR-032 dramatically increased, with 90% of the spores remaining cultivable, while B. subtilis 168 was almost noncultivable after 20 min of exposure (Fig. 7B). Similar results were observed with UVA irradiance; however, B. subtilis 168 remained cultivable after 20 min (Fig. 7C). It should be noted that it was not possible to tune the KSC xenon arc lamp in real time; therefore, when filters were placed in front of the light path, a 20 to 40% decrease in UV fluence occurred (57). This decrease in fluence partially explains the enhanced resistance to the two wavelengths during the experiment. However, even at the decreased fluences B. pumilus SAFR-032 remained more resistant than B. subtilis 168.



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  		FIG. 7. Effect of Mars UV irradiation, projected from the KSC simulator, on the spores of B. pumilus SAFR-032 ({blacksquare}) and B. subtilis 168 ({blacksquare}) 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.


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DISCUSSION
 
The goal of planetary protection as stated in NASA policy is prevention of forward and backward contamination (42). This policy applies directly to the control of terrestrial organisms contaminating spacecraft intended to land, orbit, fly by, or be in the vicinity of extraterrestrial bodies. Viking mission landers were terminally heat sterilized to decrease the risk of forward contamination of Mars and to ensure that terrestrial microorganisms did not contaminate the life detection experiments (42). However, the cost of designing and assembling the Viking landers was increased dramatically due to this requirement. In 1992 the Space Studies Board and the Committee on Space Research concluded from Viking mission data that Mars was less likely to support Earth-based life than previously thought (49a). The non-life-detection Mars landing missions, such as Mars Exploration Rovers, did not require all rover components to be heat sterilized prior to launch. Instead, NASA relied on a series of sequential chemical and physical sanitation steps to maintain the cleanliness of the Mars Exploration Rovers (9, 44).

The question is whether forward contamination of Mars will be significantly decreased by the inherent harsh environment at the Martian surface. Spores of B. subtilis have been shown to survive for up to 6 years under low-Earth-orbit conditions (15, 17). However, only shielding from UV radiation enabled B. subtilis endospores to survive under these conditions for a long time (15, 18). The solar flux at the Martian surface is considerably less than that experienced in interplanetary space (15, 46), and there is the potential that atmospheric conditions could further attenuate UV irradiation (7, 10).

Previous UV resistance studies have utilized model dosimetric strains and indicated that the limit for survival of organisms is approximately 200 J m–2 UV254 (35). A recent study examined the survival of a laboratory strain, B. subtilis HA 101, on spacecraft-qualified materials under simulated Martian UV irradiance conditions (46). The results suggested that ~6 logs of spores exposed on spacecraft surfaces under the simulated UV conditions were inactivated within a few tens of minutes under Mars equatorial and clear-sky, 0.5-optical-depth conditions. Other researchers have examined a B. pumilus strain that was isolated from a spacecraft assembly facility and reported that it maintained one of the highest levels of UV254 resistance reported for spores to date (28). Since most of the previously published UV resistance information has been based on the use of laboratory strains, predictions of the actual survival and possible adaptation of terrestrial life on Mars are limited due to the lack of robust empirical data. The same lack of data could also hamper efforts to use UV irradiation as a sterilization method if the most resistant organisms are not tested during the creation of dose standards. For example, the current standard for UV disinfection of drinking water is 400 J m–2 UV254. B. pumilus SAFR-032 requires doses of 2,000 to 2,500 J m–2, an order of magnitude greater than the standard, for complete sanitation (28).

The current study is the first study to report the abilities of a wide range of Bacillus species recovered from spacecraft and associated facilities to survive simulated Martian UV exposure. The sources of the type strains used in this study for phylogenetic comparison to spacecraft-related isolates varied from soils in France to milk (Table 1), reinforcing the widespread nature of spores on Earth. Correlations between type strain and spacecraft-related isolate resistance to UV irradiation were not observed. In some cases the type strain was more resistant to UV irradiation than spacecraft-related isolates (e.g., B. megaterium and B. cereus), but in general the spacecraft-related strains were more resistant to UV irradiation than the type strain of B. pumilus (Table 1). The source of spacecraft-related isolates (air or surface) did not correlate with UV irradiation resistance (Table 1).

Even though spacecraft assembly facilities are cleaned on a regular basis, it is evident that the resistance properties of spore-forming microbes allow them to adapt and persist in these environments. While it is difficult to speculate on the sources of spores within the spacecraft assembly facilities, soil is generally thought to be the main repository of spores in the environment (31) and the most likely vector of entry in this case. Evidence of this was obtained from the discovery of a strain of B. mojavensis in the JPL Mars Environmental Chamber Assembly Facility (Table 1). The type strain of B. mojavensis was isolated from a desert located within 100 miles of the JPL campus. B. pumilus was the predominant microbe that was repeatedly recovered from spacecraft (Viking in 1972 to Mars Odyssey in 2001) and the assembly facility surfaces (JPL and KSC) (22, 23, 54, 55) that resisted various perturbations, including UV and gamma radiation, and H2O2. There have been no reports of how this prevalent microbial species was transported into the facility or how the microbes adapted to survive in the conditions of the facility. The B. pumilus strains tested in this study exhibited no noticeable phenotypic differences. Recent studies employing genetic fingerprinting grouped all JPL spacecraft assembly facility B. pumilus isolates into three clusters (20). Therefore, it is likely that B. pumilus strains adapted over time to the conditions present in the spacecraft assembly facilities, and this may explain their elevated levels of resistance.

The data presented here indicate that spores of B. pumilus SAFR-032 are far more resistant to simulated Martian UV irradiation conditions than standard dosimetric strains are. Since B. pumilus SAFR-032 was isolated from a spacecraft assembly facility and exhibited enhanced UV resistance, it follows that any sanitation procedures involving UV irradiation should be based on the most UV-resistant microorganisms recovered from spacecraft. It is necessary to continue testing spacecraft contaminants in order to properly characterize the UV resistance of the viable bioload prior to launch.

Furthermore, during experiments in which spores of two different strains were mixed, it appeared that B. pumilus SAFR-032 spores protected the more UV-sensitive B. subtilis 168 spores. Specifically, colonies of B. subtilis 168 were not observed on plates following treatment with Martian UV irradiation for ~2 min or longer. However, when mixed with B. pumilus SAFR-032 spores, spores of B. subtilis 168 survived exposure to 5 or 10 min of Martian UV irradiation (Fig. 5). Autoclaved B. pumilus SAFR-032 spores did not protect B. subtilis 168 spores, and the inactivation curve for spores of B. subtilis 168 was very similar to the curves generated for unmixed 168 spores at a density of 5 x 105 spores ml–1. Further research is necessary to elucidate the influence of viable B. pumilus SAFR-032 spores or spore components, such as spore coat proteins, on protection of UV-sensitive strains.

The spectral output of the JPL X-25 solar simulator used in this study was different than that of the KSC Martian UV simulator used by Schuerger et al. (46), as shown in Fig. 1. Many factors can contribute to the spectral quality and output of UV irradiance lamps, including the age of the lamps, special coatings on the glass bulbs, and the chemical and physical composition of the bulbs. From the UV spectral irradiance at both JPL and KSC, it is evident that the KSC UV simulation had a higher UV flux at wavelengths less than 260 nm and was more lethal to both organisms tested. UV irradiation at wavelengths less than 260 nm has been shown to be highly lethal to microbes and coincides with the action spectrum of DNA, causing the most significant damage of the UV bandwidths (18, 27, 34). Since the KSC simulator was richer in UVC than the JPL simulator, the difference in the inactivation rates supports the conclusion that UV irradiation in the 200- to 280-nm range is the UV irradiation that is most detrimental to spores. Therefore, any attenuation of UVC by dust or ice particles in the atmosphere may greatly enhance spore survival.

In summary, the results of this study suggest that the UV environment on Mars is extremely harsh and that most microorganisms exposed to the sun would be rapidly inactivated at equatorial latitudes. However, the existence of organisms like SAFR-032, whose survival was significantly greater than that of the standard lab strain, B. subtilis 168, should be considered when workers examine the biocidal nature of UV irradiation specifically on Mars with respect to future robotic or human exploration missions. In addition, further research is warranted (i) to determine the biocidal effects of low Martian pressure and extreme desiccation on the survival of bacteria protected from direct UV irradiation, (ii) to study the effects of Mars dust and nonbiological spacecraft residues (e.g., lubricants) on the survival of terrestrial microorganisms with Martian UV fluence rates, and (iii) to determine if low levels of diffuse UV irradiation permit adaptation of terrestrial microorganisms to different Martian conditions. The research described here demonstrated that the Mars UV environment is likely to be very detrimental to the survival of microbial species from Earth, but until the other questions mentioned above can be properly addressed, we must remain vigilant in processing spacecraft for Mars to reduce the possibility of forward contamination of landing sites.


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ACKNOWLEDGMENTS
 
Part of the research described in this paper was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This research was funded by a Director's Research Discretionary Fund grant (100656-001057) awarded to K.V.

We are grateful to members of the Biotechnology and Planetary Protection group for technical assistance and for collecting the spacecraft-associated microbial strains. We thank P. Martin for assistance with Mars solar UV simulations at JPL, M. Anderson for Mars solar UV calibrations at JPL, M. T. La Duc for 16S rRNA gene sequence analysis, and W. Nicholson and G. Horneck for critically reading the manuscript. We also appreciate the help of J. Moores with running the Mars UV simulation models and the help of P. Smith with the development of the Mars UV models. We are thankful to K. Buxbaum and G.-S. Chen for valuable advice and encouragement.


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FOOTNOTES
 
* Corresponding author. Mailing address: Biotechnology and Planetary Protection Group, Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 89, 4800 Oak Grove Dr., Pasadena, CA 91109. Phone: (818) 393-1481. Fax: (818) 393-4176. E-mail: kjvenkat{at}jpl.nasa.gov. Back


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Applied and Environmental Microbiology, December 2005, p. 8147-8156, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8147-8156.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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