Resistance to high temperatures of spores from bacilli of shallow hydrothermal vent origins

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 22 T and B. licheniformis DSM 13 T ), 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 13 T , 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.