- Mercury from the environment can enter the food chain and cause health problems for humans, particularly developing fetuses and children.
- Mercury is more absorbable in the gut in some forms compared to others.
- Recently researchers have developed a probiotic bacteria engineered to express an enzyme that converts more absorbable forms of mercury into less absorbable forms.
- They hope this could eventually be used to minimize mercury exposure in humans, particularly vulnerable populations.
Heavy metals are required for cellular functions in both humans and the microorganisms found in their microbiomes. These heavy metals required by humans include copper, iron, cobalt, copper, manganese, molybdenum, and zinc.
However, overexposure to some of these metals can cause toxicity leading to health problems. Conversely, the difficulties of absorbing one of these metals — iron — through the gut is linked to anemia, which the World Health Organization (WHO) estimates affects nearly a third of women globally.
Another heavy metal that can be absorbed by the human body is mercury. Mercury can be absorbed by humans through the skin from cosmetics — such as the skin-lightening products banned in several countries in February 2023 — and also through the gut.
Mercury exists in different forms, but it is methylmercury, formed by the action of bacteria on mercury that exists naturally in the environment, that can be absorbed by the human gut. It is this form that poses a particular threat to fetal and child neurodevelopment.
How mercury impacts health
Populations that eat large amounts of fish are particularly affected by the health impacts of mercury, according to the WHO.
One notorious example of this occurred when a factory discharged chemical waste containing methylmercury into the sea in Minamata, Japan, between 1932 and 1968. It is now thought that at least 50,000 people were affected by a condition known as Minamata disease, caused by consuming excess mercury from fish caught in the area.
While bacteria are able to convert environmental mercury into methylmercury that can be absorbed by the gut, there are also some enzymes that can convert methylmercury into forms that the gut is less able to absorb.
These enzymes have been the focus of recent research led by a team based at Pennsylvania State University. The team aims to develop a probiotic bacteria that could colonize the gut and ensure this enzyme is released there.
Researchers presented the results of their study at the ASM Microbe 2023, the annual meeting of the American Society for Microbiology held on June 15-19, 2023, in Houston, TX.
In search of a mercury-fighting probiotic
Led by doctoral researcher Daniela Betancurt, the team investigated the enzyme organomercurial lyase, an enzyme that demethylates methylmercury, making it less absorbable.
The researchers initially used existing libraries of genomes of the microorganisms found in the human gut microbiome to identify a gene for the enzyme organomercurial lyase.
However, they found that the genes involved in mercury biotransformation were rare.
In order to determine which types of bacteria were most likely to express enzymes that would demethylate mercury, they exposed fecal samples from human donors to methylmercury and observed how this changed the composition of the microbiome present.
They found differences between individuals that were correlated to the amount of metal resistomes — which indicate the ability of bacteria to process metals — expressed by the bacteria in the gut. They carried out similar experiments in mice.
Finally, they identified a gene in a methylmercury-resistant strain of the nonpathogenic bacteria Bacillus megaterium that codes for organomercurial lyase, and inserted it into the probiotic bacterial strain Lacticaseibacillus.
Lead researcher Betancurt explained to Medical News Today in an email:
“We inserted a gene from Bacillus megaterium a bacteria isolated from an industrial disaster in Minimata Bay Japan. The gene provides the ability to convert methylmercury to poorly absorbed forms.”
The study authors say they have confirmed that the presence of this strain demethylated the mercury, rendering it inorganic, and removable from the body. However, the research has not yet been published in a peer-reviewed journal.
Betancurt said the next step for the research was to conduct experiments in mice to “help us quantify the effectiveness of the bacteria and examine its effects in the gut when the host is orally exposed to mercury.“
“We are investigating other metals as well, and [are] interested in understanding how the microbial community reacts to different metals,“ she told us.
“There is already a precedent for Lactobacilli to improve iron absorption and we hope to examine this in further detail in coming work,” said Betancurt.
Expert views on the impact of mercury
Dr. Kelly Johnson-Arbor, a medical toxicologist, co-medical director, and interim executive director of the National Capital Poison Center, Washington, DC told MNT: “There are multiple different forms of mercury found on Earth. Elemental mercury, or quicksilver, is a metal that exists as a liquid at room temperature. Inorganic and organic mercury are also found naturally in the environment in the soil and air.“
She explained that what is concerning is the high potential of mercury to contaminate commonly eaten foods:
“Because mercury exists naturally in the environment, it can be incorporated up the food chain into the human diet. In aquatic environments, inorganic mercury can be converted into organic methylmercury, and can bioaccumulate through the food chain. Larger fish — including tuna, swordfish, and marlin — generally contain higher levels of methylmercury than smaller fish, due to mercury bioaccumulation.”
Dr. Shilpa Dang, a gastroenterologist at Medical Offices of Manhattan, NY, and double board certified by the American Board of Internal Medicine and the American Board of Gastroenterology explained to MNT how this can affect the human body.
She said: “Mercury can interfere with numerous cellular processes, which can impair the nervous system’s regular operation. Mercury can enter neurons in the brain, reducing their efficiency as signal transmitters. It can also damage the protective covering of nerve fibers, known as myelin, which disrupts the communication between nerve cells.“
“Multiple health issues, particularly those affecting the neurological system, can result from mercury exposure. It may cause problems with a child’s cognitive development, language development, and motor development. Adults who are exposed to excessive mercury levels may experience memory loss, concentration issues, and physical weakness,” noted Dr. Dang.
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