People exposed to arsenic through groundwater or diet display dramatic shifts in their gut-friendly microbes, a study in Bengal has found, revealing previously unknown effects of arsenic on the body’s populations of protective bacteria.

Scientists have discovered that arsenic exposure changes the mix of these beneficial microbes that they say could tip the balance away from good health and raise the risk of obesity, ulcerative colitis and possibly other health disorders.

Since the 1990s, multiple studies have established that high levels of natural arsenic in groundwater across Bengal and other parts of the Indo-Gangetic plains have likely exposed millions to the toxic metal, contributing to skin lesions and certain cancers, among other health disorders. Over the past decade, US-based research groups have
shown that arsenic can alter gut microbes in mice, but evidence from human studies was sparse.

Now, microbial genomics specialist Soma Ghosh and her colleagues have found that people with long-term exposure to arsenic show a significant fall in the diversity of gut bacteria. While people with no known arsenic exposure have 246 distinct groups of bacteria, people with arsenic exposure have fewer than 180.

“We see similar changes in gut microbe patterns in people exposed to arsenic, whether or not they had skin lesions,” said Ghosh, an assistant professor of biotechnology at Adamas University in Barasat, and the study’s principal investigator. “This suggests that even those without visible symptoms may still be at risk from arsenic toxicity due to disrupted gut microbes.”

The human gut hosts trillions of microbes — an estimated 500 to 1,000 species of bacteria — that support health in many ways.

The microbes extract energy from food, enable cell-to-cell communication through substances produced during digestion, break down certain compounds, strengthen the immune system and protect against harmful microbes.

The study has found that two bacterial species, named Prevotella copri and Faecalibacterium prausnitzii, were significantly depleted in people exposed to arsenic. The findings were published in the Journal of Hazardous Materials on Friday.

“These are hallmark species of healthy Indian guts,” said Souvik Mukherjee, associate professor at the National Institute of Biomedical Genomics, Kalyani, and member of the study team. “In the population exposed to arsenic, Prevotella copri is near-eliminated,” he said.

The researchers also found a relative increase in bacterial species named Blautia luti, Clostridium saudiense and Streptococcus salivarius in people exposed to arsenic.

Their analysis suggested that the enhanced levels of Blautia luti and Streptococcus salivarius appeared to be associated with ulcerative colitis — a chronic inflammatory disorder that affects the colon and causes abdominal pain, diarrhoea and rectal bleeding. Streptococcus salivarius was associated with higher risk of obesity.

In most places in Bengal, piped water has reduced exposure to arsenic from groundwater. But scientists believe millions of people continue to be exposed to arsenic, which enters the human food chain through milk, eggs or agricultural produce.

The researchers examined faecal samples from 97 people in Nadia district, where arsenic contamination in groundwater is extensive, and 48 people from West Midnapore with virtually no arsenic in groundwater.

“Our findings provide the first evidence that arsenic can alter not just physical health but also internal microbial ecosystems, with wide-reaching implications,” Ghosh said. Microbial changes, she said, may serve as early biomarkers for arsenic exposure, even before symptoms appear.

The researchers believe these insights could open avenues for early detection and innovative treatment strategies.

“If our findings are validated through larger samples and more detailed research, it is possible that cocktails of friendly microbes could be used as potential therapy to counter the effects of exposure to arsenic,” Mukherjee said.

The study’s other authors are Sreemanta Pramanik, Arijit Chakraborty and Shubamoy Bhowmik at the National Environmental Engineering Research Institute, Calcutta branch, Shankha Nath at the NIBMG and Kunal Kanti Majumdar at the KPC Medical College, Jadavpur.