Gut Bugs Strip Mercury From Fish Without Blood Sacrifice

A newly engineered gut bacterium could soon make eating fish safer by neutralizing toxic mercury before it enters the bloodstream, according to groundbreaking research from UCLA and UC San Diego scientists.

The modified bacterium, a strain of Bacteroides thetaiotaomicron common in human intestines, effectively dismantles methylmercury—a potent neurotoxin found in seafood—reducing its absorption and accumulation in vital organs. The research, published in Cell Host & Microbe, demonstrates that this microbial approach could particularly benefit pregnant women concerned about mercury exposure affecting their developing babies.

“We envision the possibility that people could take a probiotic to offset the risk of consuming too much methylmercury, especially when pregnant,” said UCLA associate professor Elaine Hsiao, who directed the study as head of the UCLA Goodman-Luskin Microbiome Center.

The research team engineered the bacteria by inserting two genes from mercury-resistant soil microbes found in polluted mining areas. These genes produce enzymes that convert toxic methylmercury into less harmful forms that don’t readily cross into the bloodstream.

In laboratory tests, the engineered bacteria efficiently detoxified methylmercury from both pure solutions and actual fish tissue. When introduced into mice fed diets containing mercury-rich bluefin tuna, the bacteria reduced mercury levels in the intestines within just three hours.

More significantly, mice colonized with the engineered bacteria showed markedly reduced mercury accumulation in maternal and fetal tissues during pregnancy—a critical period when methylmercury poses its greatest risks to neurological development.

“By reducing dietary methylmercury in the intestine, the gut bacteria helped to eliminate it from the body before it could enter the maternal bloodstream and access the developing offspring,” explained Kristie Yu, the study’s first author and a UCLA research scientist.

The work addresses a persistent global health challenge. Despite international efforts to reduce mercury emissions since the 2013 Minamata Convention, methylmercury continues accumulating in seafood, particularly large predatory fish like tuna and swordfish, which concentrate mercury from everything below them in the food chain.

For many communities worldwide, seafood represents an irreplaceable dietary staple and cultural touchstone. “Fish remains a major and culturally important part of the diet for many people around the world and we hope it continues to be,” noted co-senior author Amina Schartup, associate professor of marine biogeochemistry at Scripps Institution of Oceanography.

Beyond simply lowering mercury levels, the researchers documented meaningful biological benefits. The engineered bacteria reduced harmful genetic and cellular changes in fetal mouse brains exposed to dietary mercury. By examining brain tissue samples, they found that maternal colonization with the modified bacteria protected against abnormal expression of genes related to cellular stress, protein translation, and cell cycle regulation—all typical mercury toxicity markers.

The approach offers advantages over conventional chelation treatments for metal poisoning, which indiscriminately bind many metals, including essential nutrients like zinc and copper. The bacterial approach specifically targets methylmercury without disrupting beneficial minerals.

Franciscus Chandra, another researcher on the team, highlighted that the bacteria were effective even with different fish types and mercury concentrations. “When we repeated the experiments with salmon, which contains lower levels of methylmercury than bluefin tuna, the bacterium was also effective,” he said.

What makes this approach particularly promising is that B. thetaiotaomicron has already completed clinical safety trials as a potential probiotic. A recent double-blind, placebo-controlled human trial demonstrated its safety, potentially accelerating the path toward practical applications.

The project received support from multiple research institutions, including the National Institute of Environmental Health Sciences, the National Science Foundation, and the Simons Foundation.

While the researchers caution that human studies remain essential next steps, they’ve begun optimizing the bacteria for greater efficacy. The approach could eventually offer a practical solution for individuals and communities that rely heavily on seafood consumption—allowing them to maintain their dietary traditions while reducing associated health risks.

For the billions who depend on fish as their primary protein source, this microbial innovation could eventually transform the risk calculation around seafood consumption, especially during pregnancy when developing brains are most vulnerable to toxic exposures.