Most of us assume scientists have mapped every corner of the human body by now. Turns out, they missed something big hiding in plain sight: massive DNA structures living in our mouths that could reshape how we think about oral health and disease.
Researchers at the University of Tokyo have discovered Inocles, circular DNA elements averaging 350,000 base pairs in length that reside in about 74% of the global population. These genetic giants had evaded detection until now, despite being among the largest mobile genetic elements ever found in the human microbiome.
A Discovery Born From Technical Innovation
Finding Inocles required solving a fundamental problem: human saliva contains 85-95% human DNA, making it nearly impossible to sequence bacterial genetic material effectively. Project Research Associate Yuya Kiguchi and his team developed a preprocessing method called preNuc that selectively removes human DNA while preserving bacterial material.
“We know there are a lot of different kinds of bacteria in the oral microbiome, but many of their functions and means of carrying out those functions are still unknown,” said Kiguchi. “By exploring this, we discovered Inocles, an example of extrachromosomal DNA — chunks of DNA that exist in cells, in this case bacteria, but outside their main DNA.”
The team used advanced long-read sequencing to capture complete Inocle genomes, something impossible with conventional methods that fragment genetic data. Their analysis of 56 saliva samples revealed these circular elements primarily inhabit Streptococcus salivarius bacteria, though pinpointing exact hosts proved challenging. When researchers tried culturing bacteria containing Inocles in laboratory conditions, the DNA elements disappeared during liquid culture, explaining why they had never appeared in genomic databases.
The global prevalence analysis using 476 salivary samples from seven countries revealed striking geographic variations. Indonesia showed the highest Inocle prevalence at 90%, while Japan had the lowest at 64%. Non-industrialized countries generally harbored more diverse Inocle types compared to industrialized nations.
Genetic Powerhouses For Bacterial Survival
What makes Inocles remarkable isn’t just their size, it’s what they carry. These DNA circles pack an impressive arsenal of stress-resistance genes, including multiple DNA repair mechanisms, oxidative stress defenses, and cell wall modification systems. The researchers identified genes for DNA polymerase III, gyrase, and various helicases that help bacteria survive the harsh oral environment.
“The average genome size of Inocle is 350 kilobase pairs, a measure of length for genetic sequences, so it is one of the largest extrachromosomal genetic elements in the human microbiome,” said Kiguchi. “This long length endows Inocles with genes for various functions, including resistance to oxidative stress, DNA damage repair and cell wall-related genes, possibly involved in adapting to extracellular stress response.”
Twenty percent of Inocle genes relate to cell wall and membrane functions, suggesting these elements help bacteria better colonize human oral tissues. The researchers found evidence of a sophisticated protein processing system involving sortase enzymes and specialized anchor proteins that likely enhance bacterial adhesion to oral epithelial cells.
The discovery carries immediate clinical implications. Analysis of blood samples from Inocle-positive individuals revealed significant correlations with immune cell populations and inflammatory protein levels. People harboring Inocles showed increased B cell activity and elevated expression of proteins involved in pathogen response pathways.
More intriguingly, patients with head and neck cancer and colorectal cancer showed markedly reduced Inocle prevalence compared to healthy controls, while those with pancreatic cancer and rheumatoid arthritis showed no such reduction. This disease-specific pattern suggests Inocles might serve as biomarkers for certain gastrointestinal cancers.
“What’s remarkable is that, given the range of the human population the saliva samples represent, we think 74% of all human beings may possess Inocles,” said Kiguchi. “And even though the oral microbiome has long been studied, Inocles remained hidden all this time because of technological limitations.”
The researchers are now working to establish stable laboratory cultures of Inocle-containing bacteria and investigate how these genetic elements spread between individuals. With 95% of Inocle genes still functionally uncharacterized, there’s potential for discovering entirely new biological processes. The team will use computational protein modeling and laboratory experiments to decode the remaining genetic mysteries.
The implications extend beyond oral health. If Inocles influence cancer risk or immune function, understanding their biology could open new therapeutic avenues. The fact that these massive genetic structures remained hidden in one of the most studied parts of the human microbiome suggests our microbial world still holds major surprises.
Nature Communications https://doi.org/10.1038/s41467-025-62406-5
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