Discovering non-toxin-producing Bacillus strains in Earth's orbit and what they mean for the future of space exploration
Imagine you're an astronaut aboard the International Space Station (ISS), floating 408 kilometers above Earth's surface. You're surrounded by cutting-edge technology, conducting experiments in the most extreme environment humans have ever inhabited. But you're not alone. Unseen companions share this enclosed space—not fellow crew members, but microbial stowaways that have hitchhiked their way to orbit. Among these space travelers exists a special group of bacteria that has captured scientific attention: non-toxin-producing Bacillus cereus strains belonging to the Bacillus anthracis clade, discovered during ongoing microbial surveillance of the ISS 1 .
Ongoing investigation tracking microbial life aboard the ISS to understand adaptation in space environments.
Future missions to Moon and Mars require understanding of microbial behavior in closed space habitats.
To understand why the ISS discovery is significant, we must first examine the complicated family dynamics of the Bacillus cereus group. This bacterial clan includes several members that, despite their genetic similarity, have very different relationships with humans:
The infamous cause of anthrax, a severe disease affecting both animals and humans.
PathogenicTypically associated with food poisoning from contaminated dairy products, grains, and spices.
OpportunisticKnown for its insecticidal properties and widely used in biological pest control 5 .
BeneficialWhat makes this group particularly fascinating—and complicated—is that these bacteria are genetically very similar in their chromosomal DNA. The dramatic differences in their effects on humans and other organisms largely come from extra-chromosomal elements called plasmids 7 . These mobile genetic elements can carry toxin genes, capsule genes, and other virulence factors that transform harmless bacteria into potential pathogens.
| Species Name | Primary Association | Virulence Factors | Plasmids Required for Pathogenicity |
|---|---|---|---|
| Bacillus anthracis | Anthrax disease | Tripartite toxin, polyglutamate capsule | pXO1 (toxin), pXO2 (capsule) |
| Bacillus cereus | Food poisoning | Enterotoxins, emetic toxin | Not plasmid-dependent |
| Bacillus thuringiensis | Insect pathogen | Crystal toxins | Plasmid-encoded |
| B. cereus biovar anthracis | Anthrax-like disease in primates | Tripartite toxin, hyaluronic acid capsule | pBCXO1 (combined toxin and capsule genes) |
In an ongoing Microbial Observatory investigation of the ISS, scientists conducted a comprehensive census of the station's microscopic inhabitants. From various locations throughout the space station—including the Kibo Japanese experimental module, the U.S. segment, and the Russian module—researchers isolated 11 Bacillus strains for detailed study 1 .
Bacillus strains isolated from the ISS for detailed study
16S rRNA gene sequence match to B. anthracis-B. cereus-B. thuringiensis group 1
When initially examined, these bacteria showed high similarity (>99% 16S rRNA gene sequence match) to the B. anthracis-B. cereus-B. thuringiensis group 1 . They shared physical characteristics with this group as well, from their fatty acid composition to their peptidoglycan type. Yet something was different—these ISS isolates didn't behave like typical pathogens.
The ISS strains displayed several key traits that distinguished them from dangerous B. anthracis:
Genetic analysis confirmed what these phenotypic hints suggested: the ISS strains lacked the two virulence plasmids (pXO1 and pXO2) that make B. anthracis dangerous . Without these genetic elements, they couldn't produce the tripartite toxin or the protective capsule that helps B. anthracis evade immune systems.
Determining the exact identity of the ISS Bacillus isolates required a multi-faceted approach, combining classical microbiology with cutting-edge genomic techniques:
Researchers collected samples from eight locations on the ISS, focusing on surfaces where microbial contamination might occur. They isolated 11 Bacillus strains for further analysis 1 .
Scientists examined the physical and biochemical properties of the isolates, including:
This comprehensive phase included:
Researchers compared the ISS strains with known Bacillus species using:
| Analysis Method | Finding | Significance |
|---|---|---|
| Plasmid Detection | No pXO1 or pXO2 plasmids present | Ruled out classification as B. anthracis |
| DNA-DNA Hybridization | 88-90% similarity to B. anthracis, but only 42% to B. cereus | Showed close relationship to B. anthracis despite lacking virulence plasmids |
| Average Nucleotide Identity | >98.5% with B. anthracis | Confirmed close genetic relationship |
| Multilocus Sequence Typing | Placed in distinct clade closely related to B. anthracis | Revealed unique positioning within B. cereus group |
The results revealed a fascinating paradox: while these ISS strains were genetically closer to B. anthracis than to typical B. cereus, they lacked the virulence machinery that defines B. anthracis as a pathogen. They represented a new clade within the B. cereus group—one that had never been described before 1 .
Space microbiology relies on specialized tools and techniques to identify and characterize microorganisms in the unique environment of spacecraft. The following table highlights key reagents and methods essential to this field of research.
| Reagent/Method | Primary Function | Application in ISS Bacillus Study |
|---|---|---|
| Whole Genome Sequencing | Determine complete DNA sequence of organisms | Revealed genetic similarity to B. anthracis and absence of virulence plasmids 1 |
| 16S rRNA Gene Sequencing | Phylogenetic placement of bacteria | Initial identification showed >99% similarity to B. cereus group 1 |
| DNA-DNA Hybridization | Compare overall genetic similarity between strains | Showed ISS isolates were 88-90% similar to B. anthracis 1 |
| MALDI-TOF Profiling | Rapid microbial identification based on protein spectra | Confirmed placement in B. cereus sensu lato group 1 |
| PCR for Toxin Genes | Detect specific virulence genes | Verified absence of anthrax toxin genes 1 |
| Antibiotic Susceptibility Testing | Determine resistance profiles | Showed resistance to beta-lactam antibiotics but susceptibility to other classes 4 |
Advanced genomic techniques revealed the unique genetic makeup of ISS Bacillus strains, showing their relationship to B. anthracis while lacking virulence plasmids.
Traditional microbiology methods combined with modern analytical techniques provided comprehensive understanding of bacterial properties.
The discovery of these unique Bacillus strains on the ISS extends beyond academic curiosity—it has real-world implications for the safety and success of human space exploration.
Spaceflight creates a perfect storm of conditions that increase health risks from microorganisms:
Occurs in astronauts during spaceflight
With limited medical resources
While the Bacillus strains found on the ISS currently lack toxin-producing capability, their close genetic relationship to B. anthracis raises questions about potential risks. In Earth environments, we know that Bacillus species can exchange plasmids through horizontal gene transfer 5 . If a non-pathogenic ISS strain were to acquire virulence plasmids from another Bacillus isolate, it could theoretically gain pathogenic potential.
Beyond direct health concerns, Bacillus cereus strains in space environments pose another threat: microbiologically influenced corrosion (MIC). Recent research has shown that under simulated microgravity conditions, B. cereus produces more extracellular polymeric substances (EPS) and forms enhanced biofilms that can accelerate the degradation of aluminum alloys used in spacecraft structures 6 .
This corrosion could compromise critical systems in spacecraft water recovery systems and other infrastructure, creating a significant challenge for long-duration missions where equipment replacement isn't feasible.
The discovery of novel Bacillus cereus strains on the ISS highlights the importance of continued microbial surveillance in space habitats. As we prepare for longer missions beyond low-Earth orbit, understanding how microorganisms adapt to space conditions—and how they might evolve—becomes increasingly critical.
Future research directions include:
The ISS Bacillus strains serve as a powerful reminder that as we venture further into space, we must bring our understanding of Earth's smallest inhabitants with us. These microbial stowaways have stories to tell about survival, adaptation, and the interconnectedness of life—even in the most extreme environments.
As Dr. Kasthuri Venkateswaran, senior research scientist at NASA's Jet Propulsion Laboratory and lead author on the key ISS Bacillus study, noted, this research 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" 1 . The humble Bacillus may well hold keys to our future as a spacefaring species.