## 1 Introduction  
Natural environments characterized by extremes in one or more physico-chemical conditions are usually inhabited by microbes, or extremophiles, possessing well-adapted mechanisms to contrast the stresses encountered in their own habitats. Extremophiles exhibiting multiple adaptative mechanisms, or poly-extremophiles, are ideal candidates for experimentally addressing questions to extend our knowledge in different disciplines, with implications for the origin and the limits of terrestrial life or astrobiological purposes, and to develop novel strategies to prevent microbial contamination [1, 2, 3]. Wet heat is used regularly for the inactivation of bacterial spores, generally at \(\geq\)100 \({}^{\circ}\)C for short to moderate periods of time. The main feature of bacilli is their ability to produce spores that characterize forms resistant to various environmental and laboratory stresses, such as chemical oxidizing agents, sterilization techniques, extreme desiccation, wet and dry heat and ultraviolet and ionizing irradiations [4, 5, 6, 3]. However, various factors are responsible for the increased resistance of _Bacillus_ species spores to wet heat, includingthe following: (i) the water content of the spore core and the level and type of the spore's core mineral ions; (ii) the high level of dipicolinic acid (DPA) in the spore core; (iii) the intrinsic stability of total spore proteins and the saturation of spore DNA with \(\alpha/\beta\)-type small, acid-soluble spore proteins (SASPs) [4; 6]. The shallow hydrothermal vents on the Eolian Islands (Italy), characterized by extreme environmental conditions such as high temperatures, low pH, high concentrations of H\({}_{2}\)S and hydrocarbons, etc., are clearly accessible sampling locations to investigate microorganisms inhabiting extreme marine ecosystems in order to gain new insights into microbial diversity and to isolate novel thermophiles, defined by an optimal growth temperature in the range of 45 \(\div\) 70 \({}^{\circ}\)C, with interesting physiological characteristics and biotechnological applications [7; 8]. It was reported that several thermophilic _Geobacillus_ and _Bacillus_ strains isolated from these environments possess a higher resistance to high temperatures than their mesophilic counterparts do [3; 9]. Spores' structural adaptation to high temperatures, such as the lipid composition of membranes, proteins and DNA, could allow thermophiles to resist extreme environmental conditions, such as those of shallow hydrothermal vents [3]. We previously reported that spores from Eolian thermophiles possess high and unexplored levels of resistance to several stresses that they have never naturally encountered before, such as UV-C, X-rays, hydrogen peroxide, low-pressure plasma, dry heat, space vacuum and high-energy charge particles [3; 6]. Interestingly, spores from _Bacillus horneckiae_ SBP3 DSM 103063 (SBP3), which were isolated from a shallow hydrothermal vent on Panarea Island (Eolian Islands), showed higher levels of resistance to low-pressure argon plasma and dry and wet heat than spores from its close phylogenetic relative strain _B. horneckiae_ DSM 23495 (BHO), which were isolated from spacecraft assembly facilities [10], and _B. subtilis_ 168 (BSU), which were isolated from a biodosimetry strain and space microbiology model organism [3]. SBP3 showed the same degree of spore resistance to wet heat treatment in comparison to _B. pumilus_ SAFR-032 and _B. nealsonii_, which were isolated from spacecraft assembly facilities and characterized by high levels of resistance to different decontamination efforts--either chemical or physical treatments [3; 10]. More recently, we reported that spores from thermophiles, including those from the SBP3 strain, were more resistant to heavy ion (helium (He) and iron (Fe)) irradiations than psychrophiles were [6].