For years, many health-conscious individuals have taken probiotic supplements – containing live bacteria, most often lactobacilli and bifidobacteria – hoping to restore balance to their gut microbiome. But a large new study published in the journal Cell Host & Microbe casts doubt on the widespread effectiveness of these supplements.
Researchers at the University of Cambridge in the United Kingdom have identified a group of previously unknown bacteria whose presence in the gut appears consistently linked to solid health. This discovery could reshape our understanding of the complex relationship between gut bacteria and overall wellness.
Study Included Participants From 39 Countries
The findings stem from an analysis utilizing the Unified Human Gastrointestinal Genome catalog, a comprehensive reference of human gut microbial genomes. This catalog identified 4,600 different bacterial species, with approximately two-thirds – around 3,000 – never before observed in the gut, referred to as the “hidden microbiome.” Until now, the function of these unknown microorganisms remained a mystery.
The research team collected stool samples from 11,115 individuals across 39 countries in Europe, Asia and North America. Some participants were healthy, while others had various conditions, including Crohn’s disease, colorectal cancer, Parkinson’s disease, multiple sclerosis, obesity, and chronic fatigue – a total of 13 health issues. Researchers then analyzed the genomes of all microorganisms present in the samples to map the composition of each individual’s “microbiome.”
By collecting and analyzing stool samples from over 11,000 people, researchers at Cambridge identified a group of bacteria previously unknown, but which appear to play an essential role in health. © Cell Host & Microbe, 2026
“Hidden” Bacteria Strongly Linked to Health
By comparing each sample to the Unified Human Gastrointestinal Genome catalog, the researchers first observed that certain bacteria, grouped as CAG-170, constituted the part of the “hidden microbiome” most closely associated with good health, across all countries studied.
To determine which species had the greatest potential to maintain balance in the gut ecosystem, they analyzed the gut microbiome composition of 6,000 healthy volunteers. The results showed that, among all the bacteria in the “hidden microbiome,” those in the CAG-170 group were most consistently associated with health.
The team then examined the gut microbiome composition of volunteers experiencing “dysbiosis” – an imbalance in the gut microbiome known to contribute to numerous diseases, such as irritable bowel syndrome, rheumatoid arthritis, anxiety, and depression. The analysis revealed that lower levels of CAG-170 bacteria in the gut were associated with a higher risk of dysbiosis.
Potential for New Diagnostic and Treatment Tools
The reason for the link between the presence of CAG-170 bacteria and good health remains unclear. According to the researchers, these bacteria appear capable of producing high levels of vitamin B12, potentially promoting the growth of other beneficial bacterial species.
They also produce enzymes that can break down a wide range of food compounds in the gut – carbohydrates, sugars, and fibers – and thus improve nutrient extraction and the production of beneficial metabolites, such as short-chain fatty acids.
The discovery of CAG-170 bacteria offers promising avenues for future research. Researchers suggest these bacteria could serve as a biological marker to assess the health of the gut microbiome and potentially lead to the development of new probiotics specifically designed to support and maintain adequate levels of CAG-170 bacteria in the gut.
“The probiotic industry hasn’t really kept pace with research on the gut microbiome,” said Alexandre Almeida, a researcher in the veterinary medicine department at the University of Cambridge, who led the study. “People are still using the same probiotic species as decades ago. We are now discovering new groups of bacteria, like CAG-170, that have important links to our health, and probiotics aimed at supporting them could have much greater beneficial effects on health.”
