Researchers from the University of California and the Scripps Institution of Oceanography have modified the genetic code of a gut bacterium, by inserting DNA from bacteria with strong mercury detoxification capabilities. The results appear to be very positive. In the coming months, the researchers will strive to establish whether the bacterium could be transformed into a safe, effective probiotic that is suitable for consumption by humans.
Fish is good for you, but some marine animals (especially large predators such as bluefin tuna) contain levels of mercury that can be toxic for the nervous system and other organs. Due to this well-known issue, dietary guidelines often state that seafood intake should be limited, especially during pregnancy. This is despite the fact that fish contains invaluable omega-3 fatty acids.
A satisfactory solution has yet to be found for removing mercury from fish, partly because large amounts of the substance are found in the sea. While some has natural origins, the majority comes from industrial waste in certain fields (such as gold mining and PVC plastic manufacturing), or improper disposal of waste such as batteries and old thermometers. In addition, a large proportion of the mercury comes from coal-burning plants. Emissions can travel through the air for long distances before settling in rivers or the sea.
Why do fish have such a high mercury content? The problem lies in accumulation
When it ends up in aquatic systems, mercury is transformed by bacteria into a substance called methylmercury, and it is in this chemical form that it is absorbed by marine animals and their predators. The average levels of methylmercury in the water are actually low, but there is a heightened accumulative effect in fish towards the top of the food chain. Feeding on smaller fish which are also contaminated, these fish build up a high mercury content.
However, it now seems that a way of dealing with toxic mercury might have been found in a field that has remained largely unexplored until now: research into microbiota.
A new hope for reducing mercury contamination
In a drive to reduce mercury contamination, researchers from the University of California (UCLA) in Los Angeles and the Scripps Institution of Oceanography in San Diego modified a very common type of gut bacteria, known as Bacteroides thetaiotaomicron, by inserting into its genetic code a “piece” of DNA taken from other bacteria that live in mines contaminated by mercury and are capable of defending themselves well against it. Large amounts of the boosted bacterium were then administered to laboratory animals (mice), along with high doses of methylmercury. As noted in the scientific journal Cell Host & Microbe, the results appear to be very positive. After just three hours, the methylmercury levels were significantly lower, and they continued to fall over the following four days.
Protection for both maternal and foetal tissues
Following the success of the initial, “technical” tests, the biologists in California sought to establish what would happen in more realistic conditions. They fed the animals in a more routine way, with a diet of bluefin tuna that had a high concentration of mercury. Once again, they got very encouraging results, which showed that much less methylmercury also accumulated in the two organs normally worst affected by mercury contamination: the brain and liver. Finally, the researchers tested pregnant mice, to ensure that the bacteria also worked in their foetuses. They found much lower levels of methylmercury in both foetal and maternal tissues.
Following this initial, experimental stage, in the coming months new tests will be conducted in an attempt to establish whether the bacterium could be transformed into a safe, effective probiotic that is suitable for consumption by humans, in order to limit the harm done by the mercury content in fish.
“We envision the possibility that people could take a probiotic to offset the risk of consuming too much methylmercury,” says Professor Elaine Hsiao, Director of the UCLA Goodman-Luskin Microbiome Center.
Amina Schartup, co-author of the paper, adds: “Fish remains a major and culturally important part of the diet for many people around the world, and we hope it continues to be.”