Scientists Solve a Decades-Old Vitamin B12 Mystery, and Viruses Are the Culprit

Vitamin B12 keeps far more of the human gut running than most people realize. More than four out of every five gut bacteria depend on it to build DNA and proteins, yet only a small fraction of those bacteria can make the vitamin themselves, and no one had ever found a gene that lets a bacterium pump B12 out on purpose. A new study published in The ISME Journal by Virginia Tech researchers may have solved that puzzle. It was not bacteria giving the vitamin away. It was viruses killing bacteria and spilling B12 out of the wreckage.

Key takeaways

• Bacteria-killing viruses called phages release vitamin B12 when they burst open a cell that had been storing it, letting nearby bacteria absorb the vitamin.
• In a lab-built six-species community, phage lysis of a B12-making strain raised bacterial diversity by up to 3.73-fold, more than simply adding B12 to the growth medium did.
• Four human gut bacteria species could only use B12 released this way, not B12 sitting in the fluid around intact, unlysed bacteria.
• The work is entirely lab-dish research with engineered bacteria, not a study of real gut samples, animals, or people, so no health or supplement conclusions follow from it yet.

Gut Viruses Burst Bacteria to Release Vitamin B12

Bacteriophages, the viruses that infect and kill bacteria, release vitamin B12 into their surroundings when they rupture a bacterial cell that had been storing it, according to the study, led by David da Silva Barreira, Hannah Poole, Rogério Bataglioli, and Bryan Hsu at Virginia Tech. The team engineered a strain of Salmonella enterica Typhimurium that makes its own B12 (a "Producer") and infected it with an active phage called P22H5. Compared with a heat-inactivated version of the same phage, active P22H5 killed most of the Producer bacteria and left the surrounding fluid loaded with B12. Left alone, without any phage, the same Producer bacteria did not leak enough B12 into the fluid around them to matter. The vitamin only became available once the phage tore the cells open.

More Than Four in Five Gut Bacteria Need B12

Out of roughly 11,000 public bacterial genomes the researchers surveyed, 86% carry genes that depend on B12, while only about a third can build the vitamin from scratch. The same imbalance shows up in the human gut, where a systematic review of vitamin B12 and the gastrointestinal microbiome has linked the vitamin's levels to shifts in which bacteria thrive and how diverse the community becomes. That matters to anyone tracking a gut issue, because a more diverse bacterial community is generally associated with better gut health outcomes, even though this particular study does not measure health outcomes itself. If B12 only becomes available to most gut bacteria when something else dies and releases it, that death, not a producer bacterium's generosity, may be quietly shaping who gets to grow.

How Researchers Tested B12 Sharing in the Lab

To rule out other explanations, the team built a series of engineered "User" bacteria that cannot make B12 and must scavenge it from their environment, then paired them with the B12-making Producer strain and phage P22H5 in co-culture. Growth of the User strain jumped only when active phage was present and only when the Producer strain could make complete B12; a Producer engineered to stop partway through the biosynthesis pathway released comparatively trace amounts, and User growth stayed flat. The researchers also ruled out methionine, another nutrient released by dying cells, as the driver, since methionine alone released by lysis was not enough to support growth. They then repeated the test with four real human gut commensals, Bacteroides ovatus, Bacteroides intestinalis, Bacteroides fragilis, and Clostridium sporogenes, all of which need B12 to grow in a defined medium lacking the vitamin. All four grew when fed phage lysate from the B12-Producer. Growth from the sterile-filtered fluid around an intact, unlysed culture of the same bacteria was negligible for three of the four species, and only partially explained by a separate, non-B12 nutrient for the fourth. In a mixed six-species community built from all of these strains, active phage lysis of the complete B12-Producer strain increased bacterial diversity, measured by the Shannon Index, by 3.73-fold. Lysing a Producer strain that could only make an incomplete precursor of B12 raised diversity by just 1.38-fold, and a Producer unable to make any B12 at all raised it by only 1.01-fold, tying the diversity gain directly to how much usable B12 the dying bacteria actually released.

Why This Nutrient Sharing Matters for Gut Diversity

Phages are extraordinarily abundant inside a human gut, with prior estimates cited in the study running as high as 10 billion viral particles per gram of feces. If the mechanism described here holds up in a real gut, it would mean phages are not just deciding which bacteria live or die, they may also be controlling who gets access to a nutrient that most gut bacteria cannot make on their own. That could help explain why a diverse, resilient gut community forms in the first place, and reviews of how bacteriophages shape the gut microbiome have already flagged phage predation as an underappreciated force behind which species rise and fall over time. None of that means a phage supplement, probiotic, or B12 pill has been shown to do anything differently because of this study. The research identifies a plausible mechanism in a lab dish; it does not test, and should not be read as validating, any specific intervention for a real digestive system.

Limits of a Simplified Six-Species Lab Community

This entire body of work happened in test tubes and lab plates, not in a mouse or a person. The bacteria involved were genetically engineered lab strains, several carrying multiple gene deletions built specifically to isolate the B12 effect from other variables, rather than the wild bacterial populations found in an actual gut. The mixed community the researchers built for the diversity experiments contained six species total, a tiny fraction of the hundreds of species that coexist in a real human gut microbiome. The phages tested were also narrow in scope, a Salmonella-specific phage called P22H5 and an E. coli-specific phage called T4, and adding that second phage, or adding a dose of the antibiotic ampicillin, blunted or changed the B12-driven diversity effect in the experiments, a sign that the phenomenon is sensitive to the kind of additional pressures a real gut would constantly apply.

What Comes Next for Phage-Driven Nutrient Sharing

The researchers draw a direct comparison to the ocean, where phages are estimated to kill 20% to 40% of surface bacteria every day in a process called the "viral shunt," recycling nutrients that would otherwise stay locked inside living cells. Broader research into how phages affect gut microbiome function suggests a similar recycling role could be happening inside the gut, just not yet measured directly there. The logical next step is testing whether phage-driven B12 release actually happens, and at what scale, inside real gut samples or animal models, something this study did not attempt. Until that happens, phage-mediated B12 sharing remains a well-supported laboratory finding, not a confirmed feature of anyone's actual gut.

Common Questions

What is vitamin B12 and why do gut bacteria need it? Vitamin B12 (also called cobalamin) is a nutrient that only bacteria and archaea can manufacture; it works as a helper molecule in reactions bacteria need to build amino acids and DNA. Most gut bacteria cannot make it themselves and must scavenge it from their surroundings.

Did this study test real animals or people? No. Every experiment described here used bacteria grown in lab flasks and defined liquid media. The researchers did not sample real gut microbiomes or test any animal or human subjects.

Does this mean gut viruses are good for your health? Not proven yet. The study shows a mechanism, phage killing releases a nutrient other bacteria need, that could plausibly happen inside a real gut, but it does not measure whether that changes anyone's actual gut health.

What bacteria and viruses were used in the study? The core system was an engineered Salmonella enterica Typhimurium strain paired with the phage P22H5, tested alongside E. coli and four human gut commensal species, Bacteroides ovatus, Bacteroides intestinalis, Bacteroides fragilis, and Clostridium sporogenes.

References

1. David da Silva Barreira, Hannah M Poole, Rogério A Bataglioli, Bryan B Hsu (2026). Bacteriophage-mediated cell lysis externalizes a metabolically valuable nutrient to broadly modulate bacterial communities. The ISME Journal. https://doi.org/10.1093/ismejo/wrag160
2. Heather M Guetterman, Samantha L Huey, Rob Knight, Allison M Fox, Saurabh Mehta, Julia L Finkelstein (2022). Vitamin B-12 and the Gastrointestinal Microbiome: A Systematic Review. Advances in Nutrition. https://doi.org/10.1093/advances/nmab123
3. Stephen C. Emencheta, Chinasa V. Olovo, Osita C. Eze, et al. (2023). The Role of Bacteriophages in the Gut Microbiota: Implications for Human Health. Pharmaceutics. https://doi.org/10.3390/pharmaceutics15102416
4. Elena Kurilovich, Naama Geva-Zatorsky (2025). Effects of bacteriophages on gut microbiome functionality. Gut Microbes. https://doi.org/10.1080/19490976.2025.2481178