Discovery in a High-Radiation Environment
New research details the discovery of a radiation-resistant Bacillus cereus strain isolated from the pool water of a cobalt-60 (⁶⁰Co) gamma irradiator. This strain, designated mrbd, survived ionizing radiation doses that are lethal to most microbes and exhibited genomic similarities to Bacillus anthracis, the agent responsible for anthrax.
The study is the first of its kind to report a viable, highly radiation-tolerant bacterium from a food irradiation facility environment. The authors emphasize that while irradiation is used globally as a microbial safety tool, persistent extremophilic bacteria may resist such control measures—posing new challenges to microbial risk management and public health safeguards.
Key Findings and Implications
Radiation Resistance Beyond Expectations
The B. cereus mrbd strain tolerated gamma radiation doses up to 5 kilograys (kGy), far exceeding the lethality threshold for standard bacterial contaminants like E. coli. This resilience may be attributed to the strain’s spore-forming capacity and to adaptive mutations in genes responsible for DNA repair and oxidative stress response.
Genetic Overlap with Bacillus anthracis
Whole-genome sequencing showed 88.4% similarity to B. anthracis and nearly 99.5% to B. cereus AH820, a strain with a poorly resolved taxonomic identity. Despite the close genetic proximity to B. anthracis, the mrbd isolate lacks the hallmark virulence plasmids (pXO1 and pXO2) and toxin genes (pagA, lef, cya) required for anthrax pathogenicity. Notably, the isolate carries a functional plcR gene, a key marker distinguishing it from B. anthracis, which harbors a non-functional allele.
Virulence and Antimicrobial Resistance Potential
The mrbd strain harbors multiple genes associated with diarrheal disease, including hblACD, nheABC, and cytK, but does not possess genes associated with emetic illness. Antimicrobial resistance genes (bla, fosB, satA) were also detected and experimentally validated, including resistance to ampicillin and penicillin.
Unique Genetic Signatures
Comparative genomics revealed numerous single-nucleotide polymorphisms (SNPs) in DNA repair and replication-associated genes, including recA, recN, mutS, and dnaX. While these mutations suggest adaptive pressure under chronic radiation exposure, further functional studies are needed to confirm their role in radiation tolerance.
Relevance to Public Health and Global Security
The discovery of a hard-to-kill B. cereus strain in a high-radiation facility challenges assumptions about microbial elimination via sterilization and disinfection. This has direct implications for food safety, infection prevention, and environmental biosecurity, especially in contexts where radiation is used to ensure microbial control.
For global health security stakeholders, the findings highlight the potential for naturally occurring bacteria to evolve traits that mimic or overlap with known pathogens, creating ambiguity in microbial forensics and complicating detection and classification protocols. Continuous genomic surveillance of environmental bacteria is critical to track such evolution and identify emerging threats.
National and Global Interest
Radiation-resistant microbial strains—particularly those that mimic high-risk pathogens genetically—represent a subtle but significant concern for public health systems, biosafety policy, and international food and health standards. Their ability to survive in controlled environments raises the need for robust monitoring frameworks in laboratories, food production, and medical device sterilization workflows.
Understanding and mitigating the risks of extremophilic microbes supports national interests in health security, bioeconomy stability, and public confidence in essential safety technologies. As the global landscape of microbial threats continues to evolve, this study emphasizes the importance of vigilance even in the most controlled environments.
Rana A.Y.K.M., Islam S., Kamal A.H.M., et al. Characterization and taxonomic clarification of a non-emetic, radiation-resistant Bacillus cereus strain mrbd isolated from a ⁶⁰Co irradiator pool. Infection, Genetics and Evolution. Vol 133, September 2025.