Scientists Solve a Vitamin B12 Mystery With an Unexpected Culprit

A virus can kill a bacterium, and in the process, release a vital nutrient: vitamin B12. This discovery comes from new research by scientists at Virginia Tech.

Bacteriophages, which are viruses that infect bacteria, can free vitamin B12 that is trapped inside bacterial cells. This sudden release of nutrients can then help nearby microbes that cannot make B12 on their own. This process can change the entire microbial community.

Vitamin B12, also known as cobalamin, is essential for many biological functions. These include making DNA, forming red blood cells, and ensuring nerves work properly. However, humans, animals, and plants cannot produce it. Only certain bacteria and archaea have the ability to make B12.

How Viruses Unlock Vitamin B12

Making B12 is costly for a cell, and only a few microorganisms can do it. Many other microbes have learned to get B12 from their environment. This reliance on external B12 is common in microbial systems found in oceans, soil, and the human gut.

This situation presented a puzzle: if B12 is expensive to make and valuable to the cell that produces it, why would enough of it end up outside those cells to feed their neighbors?

The study, published in The ISME Journal, suggests a surprising and forceful answer.

Bacteriophages, or phages, infect bacteria. They are found wherever bacteria live and can greatly affect microbial communities, including those in the human gut.

During a lytic infection, a phage attaches to a bacterial cell and injects its genetic material. The virus then takes over the bacterium's machinery to create new copies of itself. Eventually, the host cell bursts open.

The new phages escape, but they are not the only things released. DNA, proteins, sugars, and other molecules that were inside the bacterium spill out into the environment.

Bryan Hsu, a biologist at Virginia Tech, explained that the cell releases "all the DNA, the protein, sugars, all these things that the bacterial cell normally hoards so that it can continue to grow."

Phages Turn Bacteria Into Nutrient Sources

The researchers discovered that vitamin B12 can be part of this cellular release.

Hsu and his team tested this idea using a controlled system. They had one bacterium that could make B12 and another that needed the vitamin but could not make enough.

When the B12-producing bacterium stayed intact, the dependent bacterium did not grow. The vitamin was present but locked away.

Once the researchers added a phage that destroyed the B12-producing cells, the situation changed. B12 was released into the environment in amounts large enough to support the other bacteria.

Hsu noted that in their controlled system, "phage is necessary. The B12 doesn’t just leak out."

A ruptured bacterium releases thousands of different cell parts. So, the researchers needed to prove that B12 itself was responsible for the growth.

David da Silva Barreira, the study's lead author, repeated the experiment. He used a genetically modified bacterial strain that could not produce B12.

Phages still destroyed these bacteria, releasing their contents. But this time, the B12-dependent microbes did not grow.

This result showed that simply bursting a bacterial cell was not enough. B12 was the crucial resource that supported the neighboring microbes.

B12 Release Reshapes Gut Bacteria

The team then moved beyond the simple lab setup. They tested major groups of bacteria found in the human digestive tract. Again, B12-dependent gut bacteria thrived when phages broke open B12-producing cells.

The effects went beyond single species. The study found that phage-released B12 significantly changed the makeup of bacterial communities and increased their diversity. When researchers simply added B12 directly to the growth medium, the diversity effect was smaller. This suggests that the timing and location of nutrient release might be as important as the nutrient itself.

Phages are often seen mainly for the bacteria they kill. These new findings highlight another aspect of their role.

When a virus destroys one microbe, the contents of that cell do not disappear. Instead, they become resources that other organisms can use. In this way, phages might act as tiny nutrient recyclers. They transfer valuable molecules from organisms that make them to organisms that rely on scavenging.

This idea might apply beyond the gut. Cobamides like B12 are exchanged among organisms in various environments, from soil to the ocean. Previous research has also found phage-mediated B12 release in marine microbial systems.

This gives viral infection an ecological role that is often overlooked. A phage might kill its immediate bacterial host while indirectly helping unrelated microbes nearby.

For the gut microbiome, these findings suggest that viruses could influence the community. They do this not only by deciding which bacteria survive but also by changing which microbes get access to scarce nutrients.

Deep Dive & References

Bacteriophage-mediated cell lysis externalizes a metabolically valuable nutrient to broadly modulate bacterial communities - The ISME Journal, 2026