A Single Score for Gut Microbiome Wellness, Built from 8,000 Samples

Can the thousands of bacterial species in your gut be summed up in one health number? This 2024 study from a Mayo Clinic team built exactly that: the Gut Microbiome Wellness Index 2 (GMWI2), a score derived from gut bacteria alone.

How it was built

The researchers pooled 8,069 stool metagenomes from 54 case-control studies spanning 26 countries — the largest effort of its kind. Using an interpretable machine-learning method (a sparse regression model), they identified 95 microbial species that best separate healthy from non-healthy gut profiles, then combined those into a single continuous score.

“GMWI2 achieved a cross-validation accuracy of 80% in distinguishing healthy from non-healthy individuals, with high-confidence subsets reaching > 90% accuracy.”

What drives the score

Certain species consistently signaled wellness, while others signaled concern:

• Wellness-positive: Faecalibacterium prausnitzii, Akkermansia muciniphila, Roseburia intestinalis, Eubacterium rectale, and Bifidobacterium longum — many of them butyrate producers that thrive on dietary fiber.
• Wellness-negative: Fusobacterium nucleatum, plus sulfate-reducing bacteria such as Desulfovibrio and Bilophila wadsworthia.

“Sulfate-reducing bacteria such as Desulfovibrio and Bilophila wadsworthia were strong negative predictors, consistent with their role in producing pro-inflammatory hydrogen sulfide.”

The index was strongest at distinguishing IBD and colorectal cancer from health, more modest for IBS, and weak for mood-related conditions.

Why it matters

GMWI2 is a leading example of turning complex microbiome data into something a person can actually act on: a single, interpretable wellness number. It also reinforces a recurring theme in gut science — fiber-fed, short-chain-fatty-acid-producing bacteria are good company, while sulfur-producing species are worth watching.