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OPEN

received: 02 November 2015
accepted: 22 February 2016
Published: 15 March 2016

Resistance of Feather-AssociatedBacteria to Intermediate Levels of
Ionizing Radiation near Chernobyl
Mario Xavier Ruiz-González1, Gábor Árpád Czirják1,2,3, Pierre Genevaux4, Anders Pape Møller5,
Timothy Alexander Mousseau6 & Philipp Heeb1
Ionizing radiation has been shown to produce negative effects on organisms, although little is known
about its ecological and evolutionary effects. As a study model, we isolated bacteria associated withfeathers from barn swallows Hirundo rustica from three study areas around Chernobyl differing inbackground ionizing radiation levels and one control study site in Denmark. Each bacterial community
was exposed to four different γ radiation doses ranging from 0.46 to 3.96 kGy to test whether chronic
exposure to radiation had selected for resistant bacterial strains. Experimental radiation duration had
an increasingly overall negative effect on the survival of all bacterial communities. After exposure toγ radiation, bacteria isolated from the site with intermediate background radiation levels survived
better and produced more colonies than the bacterial communities from other study sites with higher
or lower background radiation levels. Long-term effects of radiation in natural populations might be an
important selective pressure on traits of bacteria that facilitate survival in certain environments. Ourfindings indicate the importance of further studies to understand the proximate mechanisms acting tobuffer the negative effects of ionizing radiation in natural populations.
Ionizing radiation is present all over the Earth. For more than four billion years, cosmic rays and the decay ofradioactive isotopes have been two of the main sources of ionizing radiation1. Low levels of ionizing radiation
found normally in nature are believed to be relatively benign for organisms2,3, although a recent review suggests
otherwise4, and in humans radioactivity has been found to be associated with some types of cancers5. Exposure
to ionizing radiation above certain levels (i.e. 1 Gy) has been demonstrated to have negative biological effects bydamaging important biomolecules, such as DNA or proteins6. Novel and unnatural sources of ionizing radiation
are of anthropogenic origin, the so called “nuclear and radiation accidents”, including events such as Three Mile
Island (1979), Chernobyl (1986), and the Fukushima (2011) disasters, radioactive waste discharges (e.g. Mayak
Complex at the Techa River, 1949–1952), or the testing of nuclear weaponry (more than 2000 tests have takenplace since the Hiroshima bomb in 1945 and 32 nuclear weapons accidents have occurred7). The sudden releaseof large amounts of radionuclides has a direct impact on the health of the surrounding human population as wellas ecological and evolutionary consequences8–12 because radioactive contamination effects on both soil and water
sources can persist for decades to millennia at all trophic levels13. During the Chernobyl nuclear power plant
disaster an area of 200,000 km2 was significantly polluted with 20 different radionuclides, and while many of these
decayed within days, others such as 137Cs, 90Sr, and 239Pu and their decay products will be present for decades tomillennia12.
The accidentally polluted areas at Chernobyl provide a suitable environment for studying the evolutionary
effects that heterogeneous levels of radiation have on different organisms. At Chernobyl, a number of negative
effects, i.e. chromosomal and cellular aberrations, and deleterious mutations, have appeared in many organisms
1

Laboratoire Évolution et Diversité Biologique (EDB), UMR 5174 Centre National de la Recherche Scientifique (CNRS),
Université Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse, France. 2Department of Infectious Diseases,
Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine, Mănăştur street 3–5,
RO-400372 Cluj-Napoca, Romania. 3Department of Wildlife Diseases, Leibniz Institute for Zoo and Wildlife Research,
Alfred-Kowalke-Straße 17, D-10315 Berlin, Germany. 4Laboratoire de Microbiologie et de Génétique Moléculaires,
Centre de Biologie Intégrative (CBI), Université de Toulouse, CNRS, UPS, France. 5Laboratoire d’Ecologie,
Systématique et Evolution, UMR 8079 CNRS, Université Paris-Sud, Bâtiment 362, F-91405 Orsay Cedex, France.
6
Department of Biological Sciences, University of South Carolina, SC 29208, USA. Correspondence and requests formaterials should be addressed to M.X.R.-G. (email: marioxruizgonzalez@gmail.com)
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www.nature.com/scientificreports/(mammals, birds, insects, and plants, reviewed by Møller & Mousseau11). Moreover, a gradient in the ecologicaleffects of radiation has been found, where places with high radiation dose showed low abundance and richnessof birds, mammals and invertebrates14–16. Although lethal doses differ among taxonomic groups9; low levels ofradiation increase the mutation rate of many organisms17–20. However, several microorganisms exhibit a wide
range of tolerance to γ -radiation4,21,22 or even show a positive tropism towards radioactive substrates23. Although
microorganisms play a key role in ecosystems, we know little about the effects of radiation on wild populations of
microorganisms24. Increasing radiation was found to lead to a decline in diversity of fungi and bacteria23,25–27 (butsee Ragon et al.24), and changes in microbial community composition28,29. Feather-associated bacterial assemblages comprises both environmental and host-specific bacteria, and an earlier study found that background environmental radiation had detrimental effects on the total cultivable bacterial communities30. The microbiota fromradioactively polluted areas thus face new and strong selective pressures from background radiation, and microorganisms living in these areas can be expected to have evolved resistance towards radiation. Here we exposedbacterial communities isolated from the plumage of barn swallows, Hirundo rustica, sampled from nesting sites
in areas close to Chernobyl with different background levels of radiation to different durations of experimental
irradiation, and we examined their degrees of resistance to radiation exposure. Since barn swallows moult theirfeathers once annually between November and March31, all sampled feathers were ca. 5 months old, and mostbacteria found on feathers must have colonized them from contaminated areas at Chernobyl, and Denmark.

Results

Bacterial isolates. We isolated 87 bacterial morphotypes: 18 from a high background radiation (2.9 μGy/h),

25 from an intermediate background radiation (0.45 μGy/h), 20 from a low background radiation (0.1 μGy/h),and 24 from a control study area with ambient background radiation (0.03 - 0.05 μGy/h). Sequencing of 16 S
rDNA allowed us to identify 74 out of the 87 morphotypes to the species or genus level (listed in Supplementary
Tables S1 and S2). Identified bacteria belonged mainly to the Bacillus-Firmicutes group (41 morphotypes) or the
Actinobacteria (27), with low frequencies of γ -Proteobacteria (3), Flavobacteria (2), and β -Proteobacteria (1)(Supplementary Fig. S1). The lowest species richness was found at high background radiation regions.

Effects of radiation on bacterial survival. To examine the effects of exposure to experimental radiation
on bacterial survival, we analysed the mean numbers of colony forming units (CFUs) for bacterial morphotypes
isolated respectively from high (14 morphotypes), intermediate (24), low (19), and control background radiation(22), which represents 67, 96.0, 95.0, and 96%, respectively, of the originally isolated morphotypes.
We found significant overall differences in survival for experimental exposure to radiation, time of exposure to experimental radiation (ANOVA, F1,628 =   913.962 and F3,628 =   44.361, respectively, P <   0.001) andintensity of background radiation (ANOVA, F3,628 =  4.846, P =  0.0024). There were significant effects for theinteraction, experimental irradiation exposure ×  time of exposure (F3,628 =  91.008, P <  0.001; Fig. 1). Non parametric Kruskall-Wallis analyses for experimental exposure to radiation and duration of exposure produced
similar results (H =  342.8 and H =  23.75, P <  0.0001 respectively) but not for background radiation (H =  2.926,
P =  0.430; the ANOSIM analysis of the interactions of experimental exposure to radiation ×  time of exposure, and experimental exposure to radiation ×  background radiation, were significant (R =  0.61 and R =  0.26,
P <  0.0001, respectively), but not the time of exposure ×  background radiation (R =  0.009, P =  0.26). The analysesof the geometric means produced similar results.
For the analysis of experimentally irradiated bacteria, we found significant overall effects for time of exposure(ANOVA, F3,314 =  91.99, P <  0.001) and background radiation (F3,314 =  4.405, P =  0.005) on the number of CFUs,
but not for the interaction time of exposure ×  background radiation (F9,314 =  0.402, P =  0.94; Fig. 2).
Effects of radiation on strains, phylogenetic group, and colony coloration. None of the bacterial
strains in the treatments without experimental exposure to radiation went extinct, while 23.36% of the experimentally irradiated bacteria perished during the experiment (Fig. 1). Thus, we found a significant effect of experimental exposure to radiation treatment effect on strain survival (irradiated vs. not irradiated; χ2 =  144.77, d.f. =  1,
P <  0.0001). There was a significant effect for duration of exposure to radiation on strain survival (χ2 =  54.92,
d.f. =  3, P <  0.0001). For the experimentally irradiated bacteria, although mortality of strains from intermediatebackground radiation was lower (18.48%) than that of other background radiations (high, 28.85%; low, 26.32%;
and control, 23.36%), the difference was not statistically significant (χ2 =  2.496, d.f. =  3, P =  0.48).
The main phylogenetic groups of bacteria in our samples were Actinobacteria and Bacillus – Firmicutes,
as stated above (Supplementary Fig. S1). To test whether differences in the composition of species affected our
results, we first compared Actinobacteria strains populations from different background radiations that were
not exposed to experimental radiation. No significant differences were found: at 2 hours, F3,24 =  1.054, P =  0.389;
at 4 hours, F3,24 =  2.301, P =  0.107; at 8 hours, F3,24 =  1.221, P =  0.327; and at 15 hours, F3,24 =  1.522, P =  0.238(Supplementary Fig. S2). Then, we conducted the same tests on the community of Bacillus – Firmicutes species,and no significant differences were found: at 2 hours, F3,35 =  0.827, P =  0.489; at 4 hours, F3,35 =  0.448, P =  0.720;
at 8 hours, F3,35 =  0.515, P =  0.675; and at 15 hours, F3,35 =  0.665, P =  0.579 (Supplementary Fig. S3). Thus, we did
not detect significant differences in the phylogenetic composition of our most common samples.
The most common colony colorations were Cream, Orange, and Yellow (39, 17, and 10 samples, respectively;
Fig. 3; Supplementary Fig. S4). We also found a few morphotypes that we allocated to other categories: White(Firmicutes, n =  4), Pink (Actinobacteria: Microbacterium oleivorans and Williamsia sp., n =  2), Feather-like colonies (Bacillus mycoides, n =  2), Shinny (Paenibacillus sp2, n =  1), and Swarming (Paenibacillus lactis, n =  1).
We did not find significant differences in the numbers of CFUs produced when creamy, orange, and yellow bacterial species were not experimentally irradiated across different background radiation intensities (ANOVA,
Supplementary Fig. S5–S7). There were significant overall differences in mortality among colonies of different
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8.0
7.0

Log CFU ± S.E.M.

6.0
5.0
4.0
3.0
2.0
Background radiation

1.0

High

Low

Intermediate

Control

0.0
0

2

4

6

8

10

12

14

16

Time of exposure (hours)

Figure 1. Number of Colony Forming Units of bacterial strains obtained from environments with different
background radiation exposed to different radiation times (CFUs ± S.E.M.). Markers for backgroundradiation in the environment are: high ( ), intermediate ( ), low ( ), and control ( ). Samples not exposed toexperimental radiation are plotted as continuous lines and experimentally exposed to radiation values arepresented in dashed lines.

Background radiation

Log CFU ± S.E.M.

6.0

High
Intermediate

5.0

Low
Control

4.0
3.0
2.0
1.0
0.0
2h

4h

8h

15 h

Time of exposure (hours)
Figure 2. Population sizes of bacteria isolated from environments with different background radiation
intensities exposed to four experimental radiation durations (log arithmetic means of CFUs + 1).
coloration across all background radiations (χ2 =  28.5, d.f. =  7, P =  0.0002) and within each background radiation (high, χ 2 =  17.52, d.f. =  3, P =  0.0006; intermediate, χ 2 =  59.52, d.f. =  4, P <  0.0001; low, χ 2 =  29.07, d.f. =  1,
P <  0.0001; and, control, χ 2 =  30.73, d.f. =  5, P <  0.0001). Among the three main colorations, Creamy morphotypes suffered the highest mortality (42.95%), which was similar to Yellow morphotypes (37.50%), whereas
Orange morphotypes had the lowest mortality (8.82%; Fig. 3). Creamy morphotypes are more frequent in zones
with lower background radiation, none and low (70%), while most of the Orange morphotypes were found at theintermediate background radiation area.

Discussion

We have found that feather-associated bacterial communities from an intermediate background radiation site
had lower mortality rates and were better able to tolerate exposure to four experimental doses of radiation thanthe bacterial communities from the other three study sites. In addition, this site hosted the largest number oforange pigmented bacteria, morphotypes that were found to be more resistant to radiation. The bacteria from thesite with the highest background radiation had an intermediate survival ability when compared with the othersites. Our results show that intermediate background radiation may have selected for more resistant bacteria(Supplementary Fig. S8).
Three potential mechanisms could explain overall resistance to radiation of bacterial strains isolated fromintermediate background radiation intensities: radiation might elicit induced responses; local microevolutionary
responses; or long term evolution. Bacteria can resist radiation through different mechanisms such as the use of

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Figure 3. Overall survival of bacterial morphotypes grouped in categories based on colony colour ormorphology. Numbers on top of bars represent the morphotypes grouped in each category (n): creamy ( ),Bacillus mycoides ( ), orange ( ), Paenibacillus sp2 ( ), Paenibacillus lactis ( ), pink ( ), white ( ), andyellow ( ).
pigments (carotenoids) or the emergence of more efficient DNA or protein repair mechanisms24,32–36. Carotenoids
are pigments that protect against oxidative stress and carotenoid-producing bacteria have been found to be associated with radioactive sites37. Unusually high DNA repair capacity has been found in the highly radio-resistantbacterium Deinococcus radiodurans38. Radio-resistance, however, seems strongly correlated with protein protection from oxidative damage, (e.g. by accumulating Mn39). Moreover, γ radiation induces the expression of certain
heat shock proteins, such as GroES-GroEL, which are able to buffer against deleterious mutations by the proper
folding of defective proteins33,40. A combination of these mechanisms may explain the performance of bacteria
from the sites with intermediate background radiation levels. For example, the presence of orange pigmented
bacteria might correlate with the presence of carotenoids that protect against oxidative damage induced by ionizing radiation. However, it might be expected that unpigmented morphotypes from these areas developed otherprotection mechanisms to minimise DNA or protein damage. The latter remains to be tested. On the other hand,
low background ionizing radiation would have little evolutionary effect on the bacteria, thus, explaining whybacteria from areas with little radiation show similar resistance to radiation to the ones with no radiation or where
the background radiation is typically very low. The bacteria that we tested have been surviving, multiplying, andevolving for more than 25 years in sites with three different background radiation levels that can be considered
low-dose radiation, that is, less than 100 mGy and dose rates below 0.1 mGy min−1, e.g. ref. 41. Moreover, adaptive
responses have been observed after doses of 1 to 500 mGy35, and effective DNA damage repair and defence pathways are activated with doses below 10 mGy41. Since bacteria lifespan ranges between less than an hour to several
weeks, the bacteria from our sampling sites that, for example, survived for a month before cell division, received
overall radiation doses ranging from 0.072 to 2.09 mGy. These doses are expected to trigger the molecular damage repair mechanisms, eliciting beneficial responses, and thus we should expect these populations to performbetter when compared to the control population. Moreover, although we did find a higher resistance to radiationin the microbial community exposed to an intermediate background radiation dose of 0.45 μGy/h, the overall
performance of this bacterial community is worse after irradiation than when it is not irradiated at all. A potential
explanation for our findings is that while many studies have been made at the individual level, we are studying theeffects of radiation at the population level across many generations. That is, low radiation doses might elicit a wide
range of protective mechanisms at the individual level, thus resulting in the improvement of survival ability even
if these responses entail cellular and molecular costs. But at the population level, the adaptive responses cannot
be evaluated because we do not have data from the original populations prior to the start of the irradiation event
caused by the Chernobyl accident for comparison of their performance. Thus, the results from our experimentmight illustrate both the costs of an adaptive response and the negative effects of radiation. In addition, it might
be possible that background radiation is not the only environmental factor driving the evolution of bacterial
resistance to radiation, and thus selecting indirectly for bacteria that are more resistant to radiation. The laterremains to be explored.

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Finally, and as expected from previous findings30,42,43, we found that radiation had an overall negative effect on
the survival of all bacteria, and that this effect increased with radiation dose. In conclusion, while little is known
about the long-term effects of radiation in natural populations, the finding of bacterial resistance to intermediatelevels of background radiation deserves further study to unveil the mechanisms adopted by these natural populations to endure the negative effects of low dose background radiation.

Materials and Methods

Ethical Statement. The research complied with requirements for research on birds in Ukraine andDenmark, and permission was given by the administration of the Chernobyl Exclusion Zone. All sampling wasapproved in an ethical review by the University of South Carolina Institutional Animal Care and Use Committee;
the methods were carried out in accordance with the approved guidelines. All birds were handled briefly, andnone died or showed signs of suffering during the short sampling period. All individuals flew upon release. The
field studies did not involve endangered or protected species.
Isolation of bacterial strains. In 2007 and 2008 wearing sterile gloves we collected eight feathers from thebody of barn swallows randomly captured as part of an ongoing long-term project on birds breeding in Ukraine
and Belarus42. Collections were made at collective farms in areas with high levels of radiation outside the southern
exclusion zone near Chernobyl and following the procedure described by Czirják et al.30. All feathers were placedin a sealed plastic bag and stored at -20 °C in the dark until microbiological analysis. Host sex ratios were balanced(N2007 =  31 Females, 29 Males; N2008 =  58 Females, 56 Males). All farms presented similar climate and habitat
types, surrounded by plantations, scattered trees, and open farmland, but were exposed to different environmental radiation levels44. The α , β , and γ radiation at ground level have previously been measured at each farm andcross-validated with measurements by the Ukrainian Ministry of Emergencies45. The two series of measurements
were strongly positively correlated30,44. We chose bird samples from places with four different background radiation intensities: high, 2.9 μGy/h (Vesniane Farm, Ukraine); intermediate, 0.45 μGy/h (Farm 49, Vetka, Belarus);
low, 0.1 μGy/h (Farm 43, Vetka, Belaryus); and, control, 0.03 - 0.05 μGy/h (Kraghede, Denmark).
In order to obtain both free-living and attached microorganisms, five feathers were sonicated at high frequency for 15 minutes in 3 repeats (5 minutes each with a 5-minute pause between repeats) in 0.8 ml of sterile
physiological (0.90% w/v) saline solution for bacterial declumping. This process does not affect bacterial viability46. After sonication samples were vortexed for 20 seconds and bacterial suspensions were transferred to a sterile
1.5 ml eppendorf tube. Then, feathers were re-suspended in 0.5 ml sterile physiological saline and vortexed again
for 20 seconds. The supernatant was transferred to a second sterile eppendorf, obtaining an overall volume of~1.3 ml solution. The method is fully described by Czirják et al.30. All samples were treated in the same way, andthus, any potential bias would have affected bacterial communities equally.
In duplicate, we spread 100 μl each of the microbial solutions on Tryptic Soy Agar (TSA, #22091, Fluka), a rich
growth medium, and the plates were incubated at 25 °C, for 3 days. To inhibit fungal growth we added 100 mg
mL−1 of cycloheximide (#01810, Fluka) to the medium. Then, we selected all bacterial colonies showing morphological differences based on colour, shape, and size, for all the sixteen plates per site, and we inoculated tubes
containing 15 mL of Tryptic Soy Broth (TSB, #22092, Fluka) medium with single colonies until cultures reachedan optical density of 0.6 at a wavelength of 600 nm. The tubes were centrifuged and the pellets re-suspended in
10 mL of TSB.
Irradiation.

Bacterial morphotypes isolated as above were randomly allocated across the wells of a 96-well
microtiter flat plate (84 isolates and three replicates of an Escherichia coli K-12 W3110 laboratory strain as control). We used 15 μl of each culture to inoculate 135 μl of TSB per well. The same inoculation order was kept to culture 16 microtiter plates that were allocated to radiation and no radiation treatments with a replicate each. Thus,
each bacterial isolate had two replicates and the control E. coli six replicates for each treatment. Three samples did
not grow well in TSB medium during the experiment and they were removed from the experiments (two fromhigh and one from low background radiation respectively).
Samples were exposed to four different doses of radiation with a BIOBEAM 8000 γ irradiator (IFR 31, Servicede Médicine Nucléaire de Rangueil, Hôpital de Rangueil, Toulouse, France) equipped with a 137Cs source todeliver radiation at a dose rate of 3.82–4.4 Gy/min: 2-hours exposure, 0.458–0.528 kGy; 4 hours, 0.917–1.056 kGy;
8 hours, 1.833–2.112 kGy; and, 15 hours, 3.438–3.960 kGy. Control plates (No radiation) were not exposed, butplaced close to the irradiator in the same room for the same period of time as the radiation treatment plates toensure similar environmental conditions.
Immediately after the treatment the corresponding plates were kept at 4 °C. Five μL of each individual culture
were serially diluted into a new microtiter plate containing 195 μL of TSB. For each sample five μL of each dilution
were spotted on LB agar plates to calculate the number of C.F.U.s per mL after 24 h of incubation at 37 °C. Overallbacterial populations were calculated by multiplying the number of CFUs for the corresponding dilution factor.
Five samples were removed from all the analysis and three more were excluded from the non-parametric pairwise
comparisons (Supplementary Table S3).

Microbial species identification. The 16 S rDNA was amplified with the universal primers fD1 and rP247.

The PCR products were sequenced in 2012 to identify the microbial species when possible and sequences aredeposited in GenBank (Supplementary Table S1).

Statistical analyses.

We calculated both the geometric and the arithmetic means of the two counts of thenumber of CFUs for each bacterial morphotype, the former to lessen the impact of dissimilar values on meaningful statistical analyses and the latter because under radiation one of the CFUs counts was zero in few cases. All

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analyses shown in the text were done using the arithmetic mean except mentioned otherwise. The means weretransformed using log (y +  1) to approximate a normal distribution. We further recorded the mortality of strains
and we used a χ2-test to examine differences. We used a three-way ANOVA to examine overall differences in thenumber of CFUs produced by each bacterial morphotype with experimental exposure to radiation (irradiated vs.
no irradiated), time of exposure (2, 4, 8, and 15 hours), and background radiation (high, intermediate, low, andcontrol) as factors. ANOVAs were performed with the R software48.
Due to the nature of the observations, however, our transformed data did not meet the assumption ofhomoscedasticity in many of the analysis. Therefore, we conducted Kruskall-Wallis tests to test for overall differences in bacteria survival for experimental exposure to radiation, time of exposure, and background radiation
intensity; and post hoc Mann-Whitney pairwise comparisons within each factor, Bonferroni corrected. Then,
we calculated 1-way ANOSIM49, using Bray-Curtis distance, to test for the interactions between backgroundradiation intensity and treatment, treatment and time of exposure, and background radiation intensity and timeof exposure. We estimated both sequential Bonferroni significance and corrected Bonferroni P-values. We used
the free software PAST50.
To examine differences in the performance of each community of bacteria isolated from sites with different
background radiation intensity within the experimental exposure to radiation treatment, we used a two-way
ANOVA with time of exposure (2, 4, 8, and 15 hours) as and background radiation (high, intermediate, low, andcontrol) as factors.
Finally, we explored whether colony colour had an effect on bacterial mortality under irradiation conditions.
To do so, first, we visually allocated bacterial morphotypes into eight different categories, established beforehandby the authors, that are based on colony colour or morphology by sight: Creamy (all those beige, pale yellow orslightly pink, and creamy colonies), Orange (all orange, peach or pale orange or peach colonies), Paenibacillus
sp1 (big pale chiral pattern-forming bacteria), Paenibacillus sp2 (translucent swarming bacteria), Pink (pink colonies), Shiny (white and very lustrous colonies), White (white colonies), and Yellow (yellow colonies). Then, wequantified the overall mortality across treatments and places for each bacterial morphotype. We used a χ2-test to
examine differences across background radiation intensity, experimental exposure to radiation, and single bacteria morphotypes across treatments when the morphotype was represented by at least 10 colonies.

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Acknowledgements

Financial support was provided by a research program of the French Agence Nationale de la Recherche (research
agreement no ANR-06-JCJC-0109-01) to MXRG, by an Eiffel grant of the French Ministry of Foreign Affairs to
GAC, by CNRS (France) to APM and PG, and by the Samuel Freeman Charitable Trust to TAM. This work is partof the ‘Laboratoire d’Excellence (LABEX)’ entitled TULIP (ANR-10-LABX-41) and was supported in part by aFrench research grant from ANR to PH (ANR-05, NT05-3_42075). We thank G. Milinevsky and I. Chizhevskyy
for support in the field and J. Mendoza for statistical advice.

Author Contributions

M.X.R.-G., G.Á.C., P.G. and P.H. designed the experiments; M.X.R.-G., G.Á.C., A.P.M. and T.A.M. conducted thefield or laboratory work; P.G., A.P.M., T.A.M. and P.H. provided equipment and materials. Everybody contributed
to the writing up.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Ruiz-González, M. X. et al. Resistance of Feather-Associated Bacteria to Intermediate
Levels of Ionizing Radiation near Chernobyl. Sci. Rep. 6, 22969; doi: 10.1038/srep22969 (2016).
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