Non-Toxin-Producing Bacillus cereus Strains Belonging to the B. anthracis Clade Isolated from the International Space Station

In an ongoing Microbial Observatory investigation of the International Space Station (ISS), 11 strains (2 from the Kibo Japanese experimental module, 4 from the U.S. segment, and 5 from the Russian module) were isolated and their whole genomes were sequenced. A comparative analysis of the 16S rRNA gene sequences of these isolates showed the highest similarity (>99%) to the - - group. The fatty acid composition, polar lipid profile, peptidoglycan type, and matrix-assisted laser desorption ionization-time of flight profiles were consistent with the group. The phenotypic traits such as motile rods, enterotoxin production, lack of capsule, and resistance to gamma phage/penicillin observed in ISS isolates were not characteristics of . Whole-genome sequence characterizations showed that ISS strains had the non- ancestral "C" allele and lacked anthrax toxin-encoding plasmids pXO1 and pXO2, excluding their identification as . The genetic identities of all 11 ISS isolates characterized via analyses arbitrarily identified them as members of the group, but traditional DNA-DNA hybridization (DDH) showed that the ISS isolates are similar to (88% to 90%) but distant from the (42%) and (48%) type strains. The DDH results were supported by average nucleotide identity (>98.5%) and digital DDH (>86%) analyses. However, the collective phenotypic traits and genomic evidence were the reasons to exclude the ISS isolates from . Nevertheless, multilocus sequence typing and whole-genome single nucleotide polymorphism analyses placed these isolates in a clade that is distinct from previously described members of the group but closely related to . The International Space Station Microbial Observatory (Microbial Tracking-1) study is generating a microbial census of the space station's surfaces and atmosphere by using advanced molecular microbial community analysis techniques supported by traditional culture-based methods and modern bioinformatic computational modeling. This approach will lead to long-term, multigenerational studies of microbial population dynamics in a closed environment and address key questions, including whether microgravity influences the evolution and genetic modification of microorganisms. The spore-forming group consists of pathogenic ( ), food poisoning ( ), and biotechnologically useful ( ) microorganisms; their presence in a closed system such as the ISS might be a concern for the health of crew members. A detailed characterization of these potential pathog