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Thermophiles from shallow hydrothermal vents (SHV) are ideal candidates to extend our knowledge for understanding the environmental limits for terrestrial life, which are also relevant in the field of astrobiology. The spore resistance of two thermophilic marine strains, Geobacillus vulcani DSM 13174 and Bacillus licheniformis T14, isolated from two SHV (Eolian Islands, Italy), to wet- and dry- heat was compared to their close phylogenetic relatives (G. stearothermophilus DSM 22T and B. licheniformis DSM 13T), and to the biodosimetry and space microbiology model strain B. subtilis 168. To determine the heat resistance, spore suspensions (107/ml) of each strain were exposed at wet-heat (95°C for 60 min) and dry-heat (130°C for 90 min) conditions. The spores viability was determined plating aliquots of each treated sample onto Tryptone Soy Agar plates, and finally the resistance of spores to both wet- and dry-heat treatments was expressed as LD90. The highest degree of spore resistance was observed for G. stearothermophilus, with similar level of resistance for G. vulcani. Spores from B. licheniformis T14 were more resistant than those of the closely related B. licheniformis DSM 13T, and also than those of B. subtilis 168. Spores of the two thermophilic marine strains were more resistant to heat stresses than B. subtilis 168, which may reflect their own adaptation to the severe environmental vents conditions. Due to their thermal resistance, the two bacilli of shallow hydrothermal vents origins may have a novel use as bacterial model organisms for further investigation into the spore responses to environment stressors, also simulating space conditions.

Microorganisms able to tolerate environmental extremes, or extremophiles, are ideal candidates to extend our knowledge on the limitations for terrestrial life, including sporicidal treatments, and on their ability to survive under conditions mimicking space environments. The spore resistance of bacilli isolated from extreme environments, cold (Antarctic soils) and hot (shallow hydrothermal vents of Eolian Islands, Italy), was evaluated towards environmental stressors (wet- and dry-heat, low and high pH values), sporicidal treatments and stresses simulating space-environments (UV-C and X-rays irradiations, desiccations by low pressure, exposition to oxidizing agents and low pressure plasma), as those from the planet surface of Europa (the satellite of Jupiter), Enceladus (the satellite of Saturn) and Mars. Spores from two thermophilic Eolian strains, Bacillus horneckiae SBP3 (SBP3) and B. licheniformis T14, and a psychrophilic Antarctic strain, B. simplex A43, were frequently the most resistant against simulating space conditions as UV-C, X-ray and heavy ions radiation. Despite belonging to the same species, spores from strains Bacillus horneckiae SBP3 and Bacillus horneckiae DSM 23175T responded differently to stresses, which may reflect the adaptation to harsh environmental conditions of the Eolian thermophilic strain. Additional comparisons were made with reference strains used in Astrobiology studies, isolated from spacecraft assembly facilities (B. horneckiae, B. nealsonii and B. pumilus SAFR032), and the biodosimetry strain and space microbiology model organisms B. subtilis 168. Spores of B. horneckiae SBP3 were able to survive stressing conditions (exposition to heat, acid and alkaline stresses, UV-C, X-rays, heavy ions, low pressure plasma, desiccation and hydrogen peroxide), often at higher degree than those of B. subtilis. The results obtained by Raman spectroscopy showed that the damages induced by heat stress to spores are related to the denaturation of proteins, rather than to those of nucleic acids. This finding suggests, that the mechanisms involved to contrast the heat stress are probably related to the thermostability of small-acid soluble proteins in spore-core. The resistance of spores to terrestrial environmental stressors could also be determinant in resisting to space stressors (as UV-C and X-rays radiations, exposure at temperature 95 and 130°C and desiccation stress). Moreover, exopolysaccharides structure and composition, involved in the biofilm formation, could be also considered as a mechanism to protect spores under stress conditions. As bacterial multi-resistant forms, the studied spores are expected to possess novel, unexplored applicative potentials in different contexts, such as new bio-indicators for safety and security in human activities, as well as in Astrobiology, in space exploration missions and “Planetary Protection” programs. Microorganisms able to tolerate environmental extremes, or extremophiles, are ideal candidates to extend our knowledge on the limitations for terrestrial life, including sporicidal treatments, and on their ability to survive under conditions mimicking space environments. The spore resistance of bacilli isolated from extreme environments, cold (Antarctic soils) and hot (shallow hydrothermal vents of Eolian Islands, Italy), was evaluated towards environmental stressors (wet- and dry-heat, low and high pH values), sporicidal treatments and stresses simulating space-environments (UV-C and X-rays irradiations, desiccations by low pressure, exposition to oxidizing agents and low pressure plasma), as those from the planet surface of Europa (the satellite of Jupiter), Enceladus (the satellite of Saturn) and Mars. Spores from two thermophilic Eolian strains, Bacillus horneckiae SBP3 (SBP3) and B. licheniformis T14, and a psychrophilic Antarctic strain, B. simplex A43, were frequently the most resistant against simulating space conditions as UV-C, X-ray and heavy ions radiation. Despite belonging to the same species, spores from strains Bacillus horneckiae SBP3 and Bacillus horneckiae DSM 23175T responded differently to stresses, which may reflect the adaptation to harsh environmental conditions of the Eolian thermophilic strain. Additional comparisons were made with reference strains used in Astrobiology studies, isolated from spacecraft assembly facilities (B. horneckiae, B. nealsonii and B. pumilus SAFR032), and the biodosimetry strain and space microbiology model organisms B. subtilis 168. Spores of B. horneckiae SBP3 were able to survive stressing conditions (exposition to heat, acid and alkaline stresses, UV-C, X-rays, heavy ions, low pressure plasma, desiccation and hydrogen peroxide), often at higher degree than those of B. subtilis. The results obtained by Raman spectroscopy showed that the damages induced by heat stress to spores are related to the denaturation of proteins, rather than to those of nucleic acids. This finding suggests, that the mechanisms involved to contrast the heat stress are probably related to the thermostability of small-acid soluble proteins in spore-core. The resistance of spores to terrestrial environmental stressors could also be determinant in resisting to space stressors (as UV-C and X-rays radiations, exposure at temperature 95 and 130°C and desiccation stress). Moreover, exopolysaccharides structure and composition, involved in the biofilm formation, could be also considered as a mechanism to protect spores under stress conditions. As bacterial multi-resistant forms, the studied spores are expected to possess novel, unexplored applicative potentials in different contexts, such as new bio-indicators for safety and security in human activities, as well as in Astrobiology, in space exploration missions and “Planetary Protection” programs.