Hiding in plain sight: rare Bacillus cereus group species are present among unassembled metagenomic samples in public databases

The soil beneath our feet is a bustling metropolis of microscopic life, teeming with bacteria that shape ecosystems, drive agriculture, and occasionally cause illness. Among these invisible inhabitants is a complex family of bacteria known as the Bacillus cereus group. While some members of this family are infamous for causing food poisoning or the deadly disease anthrax, others are harmless or even beneficial, used in industry to protect crops or produce useful chemicals. For decades, scientists have mapped the genomes—the complete genetic blueprints—of the most common and dangerous members of this group. However, a significant portion of this bacterial family remains a mystery. There are rare species within the group that were discovered over a decade ago but have never been found again in the wild. Until now, the only genetic information available for these rare species came from a single, isolated strain of each, leaving scientists with a fragmented and incomplete picture of their biology, where they live, and whether they might pose risks to human health.

The problem was not necessarily that these rare bacteria did not exist in the environment, but that they were hiding in plain sight, overlooked by traditional methods. To study bacteria, scientists usually have to grow them in a lab dish, a process called isolation. If a bacterium is rare or difficult to grow, it often slips through the cracks of these experiments. However, the scientific world has recently shifted toward a different approach: shotgun metagenomics. Instead of trying to grow a single organism, researchers can sequence all the DNA found in a sample of soil or water at once. This creates a massive, unorganized pile of genetic fragments from thousands of different organisms living together. While powerful, this method is computationally heavy, and many of these massive datasets in public archives have never been fully assembled into complete genomes. They sit there, unassembled, waiting to be decoded.

A team of researchers from Sweden set out to see if these rare, elusive bacteria were hiding within these vast, unassembled digital archives. They hypothesized that even if no one had successfully grown these rare species in a lab recently, their genetic signatures might be scattered across thousands of soil and water samples already stored in public databases. To find them, the team first identified unique genetic markers—specific sequences of DNA that act like a fingerprint—for four of the rarest known species in the Bacillus cereus group. These species, including Bacillus manliponensis, Bacillus bingmayongensis, Bacillus gaemokensis, and a species tentatively named Bacillus clarus, had each been represented by only one known genome for years. The researchers then used a specialized search tool to scan through the massive, unassembled DNA data in the public Sequence Read Archive, looking for these specific fingerprints.

The search was successful. The team identified seven metagenomic samples that contained the genetic markers of these rare bacteria. These samples came from diverse locations, including soil in an avocado orchard in Israel, the root zone of millet plants in China, and even a sample of commercial spinach that had been intentionally contaminated with a different type of bacteria for a safety study. By re-assembling the DNA fragments from these specific samples, the researchers were able to reconstruct high-quality, near-complete genomes of the rare bacteria without ever having to grow them in a petri dish. They recovered a complete genome of Bacillus manliponensis from the Israeli soil and a partial genome of Bacillus bingmayongensis from the spinach sample. This was a significant breakthrough, as it provided the first new genetic data for these species in over a decade, moving beyond the single original strain that had defined them for so long.

With these new genomes in hand, the researchers could finally compare the rare species to one another and to the few other strains that had been sequenced more recently. They discovered that these bacteria were not uniform clones; they showed significant genetic diversity. For instance, the Bacillus manliponensis found in the Israeli orchard was genetically distinct from the original strain discovered in South Korea, yet they were closely related enough to confirm they were the same species. Similarly, the Bacillus bingmayongensis found in the spinach sample was closely related to a strain found in peanut soil in China, but distinct from other strains found in Tunisia. This suggested that these "rare" species were not confined to a single, obscure location but were likely more widespread and geographically diverse than previously thought.

The study also shed light on the potential dangers and capabilities of these bacteria. The researchers examined the new genomes for genes that produce toxins or other harmful substances. They found that while these rare species did not carry the genes for anthrax or the specific toxin that causes vomiting, they did possess genes for other toxins that can cause diarrhea. Interestingly, the presence of these toxin genes varied between different strains of the same species. Some strains carried a gene for a toxin called sphingomyelinase C, while others did not. This variation highlights a crucial point: you cannot assume that all members of a bacterial species behave the same way. A strain found in a lab might be harmless, while a different strain of the same species found in a field could carry the genetic tools to cause illness.

Furthermore, the team looked at the mobile genetic elements within these bacteria, such as plasmids and viruses that can jump between organisms. They found that the rare bacteria carried unique viral sequences and, in some cases, plasmids that could potentially move genes between bacteria. One strain of Bacillus bingmayongensis was found to carry a plasmid with a gene cluster that might produce a novel antibiotic-like substance. This suggests that these rare species are not just biological curiosities but active participants in the chemical warfare and cooperation that happens in the soil, potentially holding keys to new medicines or agricultural tools.

The findings of this study challenge the assumption that these rare species are truly rare or restricted to their original discovery sites. Instead, they appear to be present in agricultural environments and food production systems, such as orchards and commercial produce, where they could interact with humans. The researchers emphasize that detecting a bacterium in a food sample does not automatically mean it is a threat; many factors determine whether a bacterium causes illness. However, the ability to find these organisms in public databases without culturing them opens a new window for monitoring. It suggests that the true diversity of the Bacillus cereus group is much larger than our current collection of lab-grown samples indicates.

By treating the vast, unassembled archives of DNA data as a treasure trove, the researchers demonstrated a powerful new way to study the microbial world. They showed that rare species are not necessarily missing from the world, but rather missing from our traditional methods of looking at it. This approach allows scientists to bypass the limitations of growing bacteria in a lab and directly observe the genetic potential of organisms living in complex environments. As more data is generated and more of these digital archives are explored, our understanding of these rare, hidden members of the bacterial family will continue to grow, revealing a more complete and accurate picture of the microbial world that surrounds us.